As for the Nuprime amp. A quick google got me the manufacturer specs of ca 80dB THD. That is ca 10dB worse as the combined performance THD plus N of the x3700 built in power amps (ca 90dB THDpN or SINAD ). So you would spent money to worsen the situation independently if it is audible or not.
So either use the built in power amps of the X3700 or if you need more power go for a bigger x model. Eg 4700 or higher.
Or if your heart is set on additional power amps look into the Hypex/Purify based amps from eg Audiophonics etc.
Or if money is no concern go for the Benchmark AHB2.
Also remember you don’t need much power for your surrounds and might use the built in amps. Center and the two mains take most of the power.
There are plenty of classic AB amps of the 80's / 90's with THD circa 70db which sound fabulous...
eg: Quad 606/707/909, Nakamichi/Threshold Stasis, Krell KSA250 - by THD / SINAD measurements these would be ranked poor - yet they subjectively and objectively (they have been involved in double blind tests at various times) - perform excellently... why?
The threshold of audibility for THD is somewhere around 60db... you can double blind test amps with 110db SINAD vs amps with 70db SINAD - and assuming that they are running within their operational parameters (ie: not driving speakers with impedances outside the amps ratings) - they all sound the same.
The key thing is that "operational parameter envelope" - external amps, typically have much larger power supplies per channel - more current is available - so when you hit a difficult speaker (eg: one with a woofer/crossover that drops impedance below 3 ohm) - they keep on going - where pretty much all AVR's run out of "puff" (current) and start to sound messy / congested, soundstage collapses etc.... (from personal experience!).
It is NOT about "Power" but about Current - a 140W@8ohm amp that shuts down when trying to drive a 3ohm load is a more "powerful" amp, than a 50W @ 8 ohm amp, which can happily drive a 1 ohm load....
In a perfect world, an amp should be able to double down into halving loads... ie: a 100W @8 ohm amp should be 200W @4ohm 400W@2ohm 800W@1ohm...... reality is somewhat less than perfect, and high current amps tend to provide something like 100W8ohm-170W4ohm-250W2ohm-375W1ohm. - the vast majority of amps cannot do this - no AVR amp that I know of can do this.
So if you have easy to drive 8 ohm speakers - with no crossover or driver driven impedance dips at any frequency - then your AVR will drive the speakers just fine - no benefit to be gained from an external amp.
But if (as I do) you have speakers where the impedance drops from 8 ohm to 3ohm on the woofer, and down to 1.6ohm on the tweeter... then an AVR is not going to be able to drive the speakers well - the impact is highly audible - even at mild SPL levels (highly audible at 72db at MLP in my setup).
Basically you need an amp that can maintain its required Voltage levels for the desired SPL's (average AND peak) - at all the impedances that your speakers throw at it... and as long as that amp has a SINAD of better than 60db (or circa 0.1% THD worst case) - you are golden!
Is it "better" to have a higher SINAD - yes absolutely... is it "better" to have a car capable of 250mph .... maybe, but would you really want a Bugatti Veyron to go do your shopping with?
And are you willing to pay the price of a Bugatti, when what you really need is a VW Golf?
I needed more power/current than the AVR provided - so I used a pair of Crown XLS2500's - SINAD measure in the mid 70's - power ratings = 440W@8ohm, 775W@4ohm, 1200W@2ohm, and just as importantly - rated to handle 1 ohm loads.... they sound a heck of a lot better than the AVR amps on their own.
(Mind you, I also tried out my Quad 606 amps, 135W@8ohm, 175W@4ohm, 90W@2ohm ... unconditionally stable into any load - did not sound quite as good as the Crowns... but a heck of a lot better than the AVR... in a double blind, might not be able to tell the difference with the Crown at lower/standard SPL's, difference may be potentially audible at higher SPL's.... maybe)
