So what do you think of Class D amp for subwoofers

You should check the damping factor of the class D amp you are considering for driving subwoofers. If damping factor is high--often in the 1k range--you should have no problem. . . If instead the damping factor were low, you may discover control problems, like with any low damping factor amp of any other technology. As for mids and treble sounding foreshortened or noisy in a switching amp. . . like with amps of any other underlying technology, it depends on the individual amp implementation. E.g. I can't perceive any obvious switching noise nor foreshortened mids/treble from my JRDG 312

Very interesting posts!

Most, if not all, class D amps put out a lot of noise starting around 15 to 20 KHz and climbs to surprisingly high levels in the ultrasonic and up into the RF range.

This noise can be filtered out if the amp is designed into a dedicated speaker. But in the case of a free standing amp, such as the unit I have, there is little that can be done about it since the designer has no idea of what speaker will be connected to it. The impedance curve of the speaker has to be taken into account when the filter is designed and therein lies the problem.

Eldartford does have a good idea to see if the Class D amp is messing with the upper ranges in the main speakers. I will have to try this now that I am curious about it. But as Sidssp pointed out, my question was more oriented toward how the Class D amp sounded in the sub bass, where it is driving the speaker.

http://datasheets.maxim-ic.com/en/ds/MAX9736-MAX9736B.pdf

Take a look at this datasheet for a Class D amp. Although this is a rather inexpensive Class D chip, you can get an idea of the noise output in the ultrasonic and RF range. Go to page 8 and look at the middle three graphs. Note how much noise is in the inband and outband graphs. This will indeed affect the sound. Just understand that the audiophile gear we buy would have better noise characteristics than this particular chip. Also, as I mentioned above, dedicated amp/speaker combos would have a filter to eliminate this. But this datasheet does give you an idea of the noise from a Class D.

Anyway, I starting to sidetrack. What I noticed about my system here is that the bass is not quite as lively sounding; rather it strikes me as a bit on the dead side. If I had to guess, the somewhat dead sound is due to the noise output.

Before I installed the 450W Marantz unit, I had four Amber 70's driving the four subs and it sounded much more musical. The problem was the 70W Amber ran out of voltage on the louder passages and clipped. Hence my purchase of the Class D - actually, I hadn't planned on buying an amp at the time but the price was too good to pass up! With 450W into 8 Ohms it doesn't clip now, that is for sure!

However, it gets worse.

Both of my tuners, the Amber/Rotel digital and the older Sansui TU-777 are both unusable when the Class D amp is on. There is so much static and interference, the system is simply not listenable. I have tried a lot so far to fix it but nothing has had much of an effect. Here is a brief list: separate AC line, huge ferrites on AC line, rca inputs, and speaker outputs, moving all the cables from the amp well away from everything else, etc. So far, nothing has helped. I did the same thing to the FM tuners, too.

My next idea is a large attic mounted directional antenna with a booster on it. I am hoping the larger signal working along with the internal squelch circuit, might block the noise. After that, my next trick is installing a large ground rod and running #6 copper ground wire to the chassis to see if that helps. I might try grounding the FM chassis, too.

I suppose I could just go out and buy four Mark Levinson, Krell, or Parasound monoblocks but let's keep it real and not dwell in fantasy land.

Anyone have any other suggestions?

I have to say that anyone claiming class D amplifiers to sound edgy and digital either have not auditioned one, or have not auditioned one blind. It's a same level of ignorance to claim tube amps sound terrible because of all the anomaly in the measurements.

Spatialking - You stated that many if not all class D amp create noise starting at 15kHz but even curves that you refer to on page 8 of Maxim datasheet shows no noise (above -100dB) up to 300kHz (carrier frequency). This noise is filtered usually with zobel network and only about 1% of noise gets out. At this frequency (300kHz) speaker cables have to be 820 ft long to create 1/4 wave antenna.
My Rowland class D amp is placed directly under TV and even with weak analog signal I cannot detect if amp is ON or OFF (not even slightest difference in noise).

Tweeters are dead quiet when I place my ear directly on it. Performance is amazing - much cleaner and liquid than class AB amp I had before.

All amps produce switching noise. Traditional amps drain current from mains in very short spikes of high amplitude repeated 120Hz.

Could you please provide example of common class D amp (Icepower, Hypex etc.) that produces switching noise at 15kHz?