Ckoffend, a single box DAC/CD player will not have a full blown tube preamp build in. It will be a DAC with (tube based) output stage and simple volume control, usually digital. In the case of the romulus the volume control is "Relay-based switched resistor analog/digital design.". This typically is one (small) board, and will easily fit into the single box.
If you are even considering going the volume controlled DAC route, I suggest you look into the Zodiac Gold + Voltikus. This is a DAC/Preamp (only one analog input) with analog volume control that has been getting some very good press.
I will be getting one myself to do a shootout against my perfectwave DAC + Modwright Preamp. I got intruiged by the DAC + Volume control scenario after (reluctantly) comparing the Perfectwave DAC (digital) volume Control directly into my poweramps, with running it through the Modwrigth. Much to my surprise, the difference were very small, and this got me thinking if I get a better DAC with better (analog) volume I should be able to get better sound than what I currently have, and resale of the modwrigt would pay for the project. A huge plus is the fact that the Zodiac has one analog intput, because this is all I need for my HT bypass.
There are some dealers that will get you a 30 day trial so trying this is a risk free proposition.