You may wish to consider a used Simaudio Moon W-5 or W-5 LE (Limited Edition) dual-mono (fully differential) power amplifier.

Would finding out (after you buy the amp) that the receiver preamp was in large part responsible for the sound degradation bother you or do you already have future plans to upgrade the control section? I ask only because you are passing the signal through a lot of circuitry through a receiver and that could be the source of your distortion at higher levels.

I would strongly consider retiring the NAD and look at integrated amps from Musical Fidelity or Krell. Either would provide plenty of current for your speakers and are well within your budget as new.

RG

I did a quick search on the specifications of the Thiels and he is absolutely correct in identifying his major problem...current.

The Thiels drop down to 2.4 ohms, though the specs didn't identify which frequency or frequencies this dip occurs. Also, rated at 86 in sensitivity certainly qualifies these speakers as difficult to drive.

Given that he is experiencing the congestion with 125 watts of power and that 250 watts would only increase the decibel ouput by 3db, I'm not entirely sure that would be enough, though I think it a fair assumption that a higher quality 125 watts that is able to double in a 4 ohm load would do the trick.

It sounds like the NAD is providing a bit of higher current, but for far too brief a period and that what he needs is a sustained 200 watts or better.

Perhaps a bridged pair of monoblocks would be the safest bet?

Krell-400xi
200W - 8 Ohms
400W - 4 Ohms

Musical Fidelity A5
250W - 8 ohms
400W - 4 Ohms

I realize he has higher aspirations and though the Levinson he's considering I'm sure is a fine amp and can be had for a song on the used market...it IS ten years old! However, he did ask for AMP suggestions so pardon me for straying, but sumthin's gotta happen with that NAD front end!

RG

I can't imagine that not being enough juice and I'm sure ditching the receiver at some point in time is a good idea. I think the Krell integrated had concerns for JA with inadequate heat sinking when driven hard, but that might have been the previous generation.