What DAC should I buy?

The MSB Nelson (and above)DAC's accept the balanced input that your Monarchy DIP provides. Stereophile loves this combo. The MSB DACs actually use a SEPERATE lower quality DAC for HDCD, and HDCD cannot be used with upsampling. I prefer upsampling to 24/132 to HDCD. If you want comparable quality in your HDCD, you'll need to look elsewhere.

Birdland models - the Lite - the Odeon, etc.....

Theta DSPro Basic IIIa with HDCD option installed is generally available used in your price range. I have the regular version of same w/o HDCD (paid $700 used) and run it with the Monarchy DIP 24/96 to good effect, but I have not tried the newer "upsampling" converters such as the MSB in my system (nor, of course, do I own any HDCD's). The Theta is a well-known, well-made, and highly-rated current-production model from a leading digital manufacturer that lists for around $2,800 new, but that seems to have fallen from favor price-wise on the used market because it lacks the latest buzzwords and has been around for a while, making it something of a deal. I have wondered how my unit would stack up against some of the newer models available for around $1,000 used (MSB, Bel Canto, Musical Fidelity, Perpetual Tech, etc.) but haven't been at all displeased with the sound I'm getting (with a Theta Pearl transport), so I've lacked motivation to check this question out for myself. Good luck with the search.