In a similar situation I ended up keeping my SV212. I wasn't going to get much on a trade in so I opted to keep it as a spare amp and also use it as an extension cab for gigs. Love the DT25 though. Keeping the SV also let's you play around with a true "stereo" set up from your POD. Although not ideal as with a pair of DT's, you can make it work and it is fun to experience. I realize however, that economics may require that your SV needs to move on either on trade or private sale. I will admit that my SV doesn't get used all that much but when my friends come over to play they don't have to bring an amp.

Sadly, I can't comment with authority on the used price value of the SV's. eBay is usually my bench mark. Do a search for "Spider Valve" (sort by Highest Price First to put the amps at the beginning of the listings) and a bunch of them will pop up both MKI's and II's