I've gotten a lot of information from the standard "preamp shoot-outs" in other forums on this site.
It wasn't until I actually experienced it myself first-hand that I became a convert to using high quality preamps with my synths. I bought a UA Solo 610
awhile back for vocals and the odd single mic acoustic recording and then one day tried it out as a DI box on one of my analog synths. Wow. What a sound!
I wouldn't use that preamp on every track for sure, but it has a lot more character and color than the neutral (but adequate) preamps on either my Mackie VLZ3 mixer or MOTU 8pre
I then invested in a UA 4-710d
which has both solid-state and tube preamps or a blend of the two. Synths sound fantastic through this as well. What's nice about the 4-710d
is that it has a good quality A/D converter onboard which can be light-piped elsewhere via ADAT.
I'm not necessarily recommending these model preamps, but rather only that IMO there is something to be gained from using higher-end preamps than what one normally finds on your average audio interface. The landscape is changing, however, as audio interfaces continue to improve. The need for expensive outboard preamps for synths is slowly diminishing it would seem.