DAC suggestion to pair with MC275 & Usher Spk

Most of the modern tech DACs are SS so if you wish to have multi-format capabilities, USB input or multiple digital inputs I'd recommend Mytek 192(can be used as digital preamp as well) or Antelope Zodiac. Both of them are available at major pro audio retailers such as sweetwater, vintageking. You will not find any better accuracy and precision in tube audiophile market.

Tube DACs are just the same as SS that normally have an inverting tube output stage to color the sound with tubes that you don't realy need since you already have MC275.

mmmm!!!
You got it!

A sub or even two.

I'd plug 2x Vandersteen 2wq. It's inexpensive and very precise sub ever made. I consider that the best value sub.

You also can get tube-friendly floor-standers instead of your monitors, but bass will not be any deeper than with sub.

Certainly the speakers you have aren't easily 'digestable' by tube amp, but integrating them with sub can give you a possibility to keep tubes. If you aiming for more control, than certainly getting MC252 or MC352 will justify your goals(if we speak in terms of Mac)
Financially and logically adding 2x2Wq is a lot cheaper. Having MC275 is more like investment that will never lose the value just like gold so I would think forever before I would decide to sell this unit.