I own a TH500, DB750, and a DB751. They all have the inherent Aguilar sound (which I describe as being a modern-vintage tone that's tube-like with a round bottom and punchy low mids, but with extra clarity and crispness in the high end and high mids), but they all do it in their own unique way.
TH500 - A awesome SS head that weighs in at something like 4lbs. You put this amp in a bag and forget it. Due to its drive knob and sweepable mids, its user is given a huge tonal plate to play with and shape to their own desire. It's perhaps the most capable of getting a modern tone. It's a good sounding amp, but the tone does not have the heft or three-dimensional aspect that its big brothers have.
DB750 - I call this the SVT killer. It does everything you'd expect an SVT to do, but much better. With a simple tube roll in the v1 position, you can have an amp with a tone that fills out an entire bands' mix with it's punchy low in and smooth, put prominent, low mids. The high end provides modern crispiness that helps your basslines come out and become more audible within the whole mix. For anyone doing rock, funk, gospel, or r&b, this is the head to judge them all against.
DB751 - This is my lease favorite of the 3. They changed the sweep in the mids to affect the higher register instead of the low. A tube roll might even this out a bit, but I have yet to try it. As it stands, the amp has a pillowy and woolly low end that gets muddy in the mix. If you take it down too much the low end completely drops, but with it cranked any its simply a muddy mess. I have yet to find the right balance between the lows and mids to make it work in my band. The high end is also clinically crisp and bright. Supposedly the amp uses an active EQ section instead of a passive. This makes sense as I tend to need to cut all my knobs back just so I'm not overpowering my speakers. Again, I need to try a tube roll, but a stock 751 is inferior to a stock 750 IMHO (most others seem to disagree, but having played 75+ gigs with each I feel pretty confident saying that.
Hope this helps!