On my AP, SMPTE IMD is limited to about .001% best case, that's the residual of the generator in SMPTE mode. Those IMD sweeps are close to the AP's residual. The better IMD test is the CCIR IMD test. That one is able to factor out of the measurements much of the residual noise which is included into SMPTE and THD measurements. CCIR IMD sweeps read to the residual of the AP, around 1 ppm.
Also, higher THD measurements at 1 watt are also including noise. The noise is constant, the power output is not. Once one tests THD with sigificant output, the noise floor is pushed further out of the measurements. This is why many pieces of gear show rising THD at lower measurement levels.
Not sure if this has been mentioned before; but the fact is not ALL amps respond the same with a speaker load compared to a resistive load.
Read the Spectral Contamination article by Deane Jensen and you will see what I mean...
Far more going on than simply connect a speaker to any amp and it SOUNDS the same...
I've been working out of a few studios in central London for the past few years and I have been unfortunate to never hear a pair of NS10's on a Bryston sounding really good. Don't hang in the right circles I suppose. I've been running my NS10's on an Alesis amp for the past few years and I've used them quite frequently. The studio I most often work out of uses an Alesis amp so I was quite used to the sound of them.
After reading some forums on Gearslutz I decided that I should get an amp that was recommended by some of the slutz to be a good match for NS10's as I wasn't completely happy with my monitoring situation. Despite having 3 sets of speakers to chose from already. I wasn't really convinced at the time that it would make any difference but it would free up another amp for me to use elsewhere.
As I can't afford a Bryston or anything in that league I decided to go for what one member said was an old British standard, the Quad 405-2. The one I got had been modified during the late 80's by a guy called Peter Martelli who at the time was working at Sheen / Cutter studios (later to be sold to AC/DC). I re-capped the amp and removed some of the passive mods. It is pretty much a Quad 405-2 now but with a few adjusted resistors and caps.
I plugged the amp in at 5am and had a listen at low volume. The results were so good that I had to text one of my engineer friends to come around and check I wasn't going mad. It dramatically changed the bottom and and smoothed them out completely. So much so that I've started working almost exclusively with them. The amp itself had been flight cased and has joined my mobile gear (mics and such) so that I can take it along to any mixing sessions I'm doing.
The improvements between these two rigs are so dramatic there is really no need for an A/B test. The amps and speakers seem to just be made for each other. I've already got 2 orders for me to build similar amps for other studios. Feels like a curtain has been pulled from in front of my monitors.