best sub $1000 amp to mate with tube preamp

I agree with Csericks (Customer Service Engineer Ricks?) and Tvad. Assuming what ever you'll be looking at will be used, it's difficult to better the sonic virtues of class D amplifiers at this price point. I drove my 4ohm, 87dB sens. Avalon Eidolons with a PS Audio HCA-2 with ease.

On the linear amp side for $1100-1400 I would suggest the Ayre V-3 for its ability to dive difficult loads and its wonderful presentation.

Very brief HCA-2 primer.

PS Audio has a history of designing decent sounding traditional components using parts that are commensurate with their cost. Away from the seen for a few years they returned with the HCA-2 one of the first of a new generation of switching amplifiers. Stereophile gave the difficult to measure $1695 amplifier a class A rating and created an instant controversy. While it doesn't sound like solid state or tubes it does have attributes of both and some of its own.

Other notable manufactures began producing, what is now termed class D, products giving credibility to low cost high performance amplification. As with some manufactures, PS Audio is marketing their second or third generations. The HCA-2 became popular for its price new and now for its value as a used amp. Some simple modifications take its output to even more satisfying levels. It's become one of those affordable stepping stones into high end sound.

If you buy one, when the time comes you shouldn't have a problem selling it.
I almost sold mine but for its current value I think I'll just hold on to it.