looking for a DAC that will add some bottom end

I find I am not happy with the bottom end of my digital. Any suggestions for a DAC? tube or SS. Used $500- 1200

Aragon D2A mkll with its elegant outboard power supply delivers the type of extended forceful bass you are seeking.
Although mine is not yet for sale (hard to part with) there is one for sale from another Audiogon member.
As an extra feature it can properly decode a specially mastered group of 1980's CD's which benefited by having a complementary automatically triggered de-emphasis circuit. It refined the painful digital treble of those early CD's, and is great for those who retained their 1980's CD collection.
Mitcho - I suggest you try some ball bearing type footers and danmping ( sand in bags - or some kind of weight over cloth ) on top of your current DAC first - worth a try
If you must try something new the Monarchy Audio M24 or NM24 (new version offering both tube and solid state outputs) are quite good in the bass region.
REL subwoofer.
Bryston BDA-1. The bass you're looking for. But difficult to find used....
PS Audio Digital Link III has incredible bass; as they claim "Subterranean bass response" - and it is not unfounded from my experience with it.
If you want not only deep base but 'articulate' base, get a PS Audio DL3 with a Cullen Circuits mod. It not only resolves lower frequencies better than the stock, it also opens the air around the bottom end better. You'll hear the wood of the kick drum instead of a dull thud. You'll hear the resonance of the strings on the base instead of a simple low pitch rumble.

It runs you about $1,500 new. While it is more than you want to spend, you won't find equivalent bottom end performance for under $2,000.
The bass is incredible on the Cullen. Not only is it deep and strong, but it also does exactly what Shazam above says. It articulates the bass, opening it up and resolving all the resonance, the wood sounds of the doublebass or the drums. Not a thud that just dies, but a sound that rings and resonates and sounds like the real thing. It's hard to give this up once you experience it.