Single input tube preamp

Linear Tube Audio MZ2 would seem to fit the bill, but you really need to add their linear power supply to get the best from it.

https://www.lineartubeaudio.com/products/mz2mz2-s

@soix 

Very interesting. I'm currently using a Schiit Valhalla 2, another OTL headphone amp/preamp in a similar setup. It gets input from my Bitfrost 2/64 and I run it wide open into a Schiit Kara preamp in passive mode. So, there's no attenuation to the tubes and the only volume control in the circuit is the Kara RL volume control. Sounds pretty nice for the cost and is what prompted me to look for something similar.

This look like the perfect upgrade but a new MZ2 is above my budget. Maybe used ones come along. I'll keep an eye out. 

Well executed, high performing simple designs. Add enclosure and attenuator. VTA Roy Mottram's SP14 is based on the same Akido circuit.

https://glass-ware.store.turbify.net/listheam.html

@blueeagle79 is referring to this 

https://glass-ware.store.turbify.net/paraikocster.html

There's an LTA Microzotl with two input tubes on USAM in your price range, and it comes with a linear power supply. The seller seems to have a great reputation. (I am not him/her.) That might be your answer. Very solid and well regarded product.