Really pleased to hear that you;re enjoying them. My advice for now would be to forget an amp (you really don't need it with the MS1i), and if you really want to experiment with sound/comfort - change the pads first (Senn 414 pads quarter modded for cheap option, or try L Cush [aka bowls]). They may help with comfort (opinions vary) and will also have an impact on sound as well. Whatever you do - do not get the Jumbo pads (tried it - not good result).
When you eventually start thinking about your next head-phone purchase - then see if an amp required - and get advice on which amp would be best suited for your particular needs. The E17 sounds like a great entryway amp - but you don't want to buy now, and then find that your needs have changed by the time you purchase your next headphones. All you'll end up with is buyer's regret and a lighter wallet.
If you need a hand with modding at any stage - feel free to PM me, and I can show you sone of the resources I used.
If you like blues - and really want a treat with the MS1is - see if you can get to listen to:
Joe Bonamassa - "Live From Nowhere In Particular" (my favourite tracks are Mountain Time and Sloe Gin) and/or
Beth Hart - "Live From Paradiso"