Should I get an Aguilar DB 680 preamp and run it with a QSC or Crown power amp or should I get an Ampeg SVT-4PRO?
I'd say Aguilar All the way. Although I think that Ampeg make great products...I think Aguilar stuff is in another world. But the costs usually associated with them are too...
no contest, for sheer hifi, gut punchin', sweet bass tones, the Aguilar's all over the Ampeg. IMHO, there's nothing in the Ampeg arsenal that remotely touches the tubey goodness the preamp has. but for the money, cant go wrong with the Ampeg. the Aguilar preamp alone is $1600, and that's a ton of dough.
This is definitely apples vs oranges. What sort of music are you playing? What sort of sound are you looking for? What's your budget? Given the choice, I'd choose Aguilar - but my requirements / budget are almost certainly different from yours. Ampegs are great, but to my ears the DB680 is the best preamp I've heard. However, if you're looking for the Ampeg sound then you should go Ampeg. As Joker said, there's quite a substantial price differential - best price you'll get on a DB680 used is around $900.