Hi Robert,
You raise an interesting and important question. Most people seem to think that a good speaker system will perform as specifications would imply, in any room and setup. The fact is that what you hear in real situations is always a complex interplay between the speakers and the room they are sitting in. This applies irrespective of whether or not you are near field monitoring or not because the distance you have to be from the monitors for the room to not affect what you are hearing is much closer than most people think (definitely less than 50cm).
In short, what you hear is a combination of the direct sound from the speakers and the ambient reflected content from the room. Now for everything to sound spectrally correct what needs to happen is that the spectral content of the ambient component has to be near flat and the polar response of the loudspeaker near flat too. If one or the other isn't then you'll end up with a strong colouration. If the polar response annomalies just happen to compliment and approxiamtely cancel the ambient resonances of your room then a technically inferior loudspeaker will actually sound better than a more expensive and superior one. I suspect this is what is happening in your situation. By all means, sell your MSP20s if you are satisfied with what you get with your JBL's but I strongly suspect that you may have some real acoustic issues to deal with if you want a truly natural and pleasant sounding system to work with. Your comments seem to suggest to me that the Yamahas are probably worth the money you paid for them but the room is colouring what you hear.
Optimising room acoustics is a challenging issue for anyone to deal with. But if you truly want good sound you have to deal with it all the same. Good acoustics rarely happens naturally. It has to be designed for and each situation is unique, requiring custom solutions. It need not be hugely expensive and when you have them sorted you will be amazed by the difference in sound clarity and suprised and how little it cost you (provided you're willing to do some work on it yourself that is). You don't need high tech materials and fabrication techniques, you don't need Sonnex or some other treatment. What you do need is a means on measuring your room response (you can do that with Har-Bal and a good instrumentation Mic - try Behringer ECM8000 for an inexpensive one) and the means of building some absorbers, which may range from drapes and curtains to something a little more complicated to deal with specific resonance issues.
My room and my speakers are a good case in point. The speakers sounded very accurate but at the same time very difficult to listen to in my less than optimal room treatment case. The main problem was a 6dB resonance imposed by the room at 3kHz which really coloured the ambinet response and made it very fatiguing to listen to anything for an extended period. At one point I was blaming my speakers but after I did some measurments I realized that it was a room issue. My first attempt at treatment went part way to solving the issue but it was not optimal by any means, and ended up creating a whole in the mid band. My second attempt is pretty much optimal and sounds it too. It took me a year to get it right but in terms of the actual time spent fixing and building what was necessary, it would only add up to a week or two of work. In terms of material cost I would have spent no more than $1000 AUS dollars (500 EU) and that is including my first absorbers which were dismantled and re-built to a different design.
What I would suggest to anyone buying gear is that they should be prepared to spend as much money on treating their room as they would on the speakers. That's not to say that you will need to spend that much (you may be blessed with a room that needs little attention), but you should be prepared to spend that much if it needs it. If not, then your investment in speakers will have been wasted. The other thing you should keep in mind is that the key specification to your loudspeakers is the uniformity of the off axis polar response (in vertical and horizontal planes). This thing is rarely if ever documented in loudspeaker specs but you can generally infer it from a few other specs. Basically what you are looking for in a two way system is a crossover point around 2kHz, a LF driver no bigger than about 7" and a crossover slope of at least 18dB/oct. The drivers should be mounted close to one another on the baffle. For a three way system the upper crossover point should be around 3-4KHz and the crossover slope the same. My speakers are essentially like a hybrid 2-way and subwoofer system. The crossover point is 2kHz using a 3rd order crossover and the low-mid and mids are covered using two 5 inch woofer mid range units from 120Hz to 2kHz. This gives them a very good horizontal dispersion that makes them sound mid-range rich in an average room, but the accuracy of the important mid range is very good owing to the small driver size. The low frequencies come from a bandpass enclosure where a 6 inch woofer is entirely enclosed in the box and the sound eminates from a 50mm diameter vent about 50cm above the floor. That covers an octave from 50Hz to 120Hz with a roll off slope similar to an infinite baffle arrangement. Having the extra octave below 50Hz is not really necessary in most situations because the wall reflections (ie. radition into something between a half and quarter space) provide adequate lift to fill this hole.
In my opinion, what you should probably steer clear of is any speaker whose central tentant is linear phase or phase coherent response (unless it is an active speaker system) because this means it will be using a first order crossover. That means there will be a huge overlap in driver output so the horizontal plane polar response may well be good but the vertical plane will be bad. This will colour your room response. I have a friend who has a pair of Duntech Marquis, a 150W Perraux intregrated and a 200W Perrraux power amp in a bi-amped arrangement which I've heard. It sounds reasonable but the total cost of the gear is around $15000 AUS and I prefer my humble $3000 AUS system anyday. In terms of clarity there's no comparison. Now most of that is the treatment of room acoustics, but part of that has to do with the design philosophy that Duntech follows, which in my opinion, just doesn't work in real rooms. The vertical polar response is attrocious which colours the ambient field.
I think that's enough rambling for one day. I recently read an excellent article on getting the best out of your speakers. Here it is.
http://www.harmanaudio.com/all_about_au ... _rooms.pdf
Have a read and think about what you could do to improve your acoustic set up. Once you gone through the easy stuff then go about measuring the room response and think about how you might optimise that. One of these days I'll document what I did to treat my room and make it available to anyone who's interested.
Regards,
Paavo.
PS - Be sure to read this if you haven't already.
http://har-bal.com/ipw-web/bulletin/bb/ ... .php?t=244
or are the Yamahas turkeys. 
