Working in reverse order, the pricing of NAD simply reflects the policy of its Canadian owner/distributor, which also owns PSB speakers. They seem to never discount or if so, by very little.
The same used to be true of H/K gear in Canada, where the distributor seemed to price it much higher than the same components in the US, even taking into account the difference in the currency and generally higher pricing in Canada (population size one-tenth of the US, more expensive doing business, costly shipping, etc. etc.)
A couple of things on the NAD power ratings. First, ignore "dynamic power", which isn't a standard at all and is basically a bogus rating, a way to inflate power output ratings with no regard to distortion or the duration of the "dynamic power." On the other hand, "dynamic headroom", rated in dB, is a real standard. NAD amplifiers often have about 3 dB of dynamic headroom, which means they'll produce twice their rated output for a long enough period to handle brief musical peaks on recordings of wide dynamic range.
The NAD amps typically have a couple of sets of voltage rails; one set is for higher power output on a momentary, short-term basis, a matter of milliseconds (NAD may specify this somewhere). And that's fine for brief musical peaks. So your NAD realistically likely has a bit less than 3 dB of dynamic headroom if its "short-term" power output is specified at 90 watts.
The H/K, if it's rated at 150 watts continuous output into 4 ohms, both channels driven, from 20 Hz to 20 kHz, with less than blah, blah THD, then it should be considered the more powerful amplifier. It might even have a smidgen of dynamic headroom, but many manufacturers don't rate or measure this aspect of performance, preferring the bogus "dynamic power output" spec instead.
Overall, I'd say No, the NAD is not worth the premium over the H/K, and I'd wager that if you put a spectrum analyzer on the output of the NAD during its peak dynamic headroom swings and on the H/K at the same output levels, the distortion of the H/K would measure lower than the NAD's. As to whether that would translate into cleaner sound, I doubt it, but I'd still go for the more powerful amplifier at the lower price--if it's measured as I outlined. Even at 8 ohms, the H/K is more powerful than the NAD.
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