I would expect the Master on a DB751 to work just fine to lower the volume.
There are plenty of lower power tube amps, but they are not voiced like a DB751. Ashdown has made several models. There are various versions of the LB30 and CTM30. Also the Little Stubby and LB30 2N. I own a Mk1 LB30, Mk2 LB30, and a Little Stubby. Each has a unique voicing. My guess is the Ashdown LB30 Mk2 (UK Custom Shop), will be closest to the DB751. It has really big lows, which is a DB751 hallmark. The overall voicing is not exactly the same though
The CTM30 was said to be the same as the LB30. But what I have found the Mk1 and Mk2 LB30 have different tone stacks and different voicings. Also the Mk2 has a choke in the power supply that I believe changes the amount of headroom in the preamp. The Mk1 preamp distorts easier, and the Mk2 is clean and more HiFi sounding. There are various versions of the CTM30 as well. I don't know if the match the voicing of the LB30 Mk1 and Mk2, or if they have their own mojo going on.
Ampeg also offers some small tube amps including the PF20T and PF50T. I own a PF50T and it is very Ampegy.
Aguilar made a DB359 that is 200W which should be voiced about the same as the DB750. So slightly different than the DB751. The DB359 was built into a very deep 2-rack space package. It's fairly heavy and an awkward carry. Also, it's still a fairly loud amp.
Good luck!