My opinion which I have developed by trying numerous transports, DACs, and digital cables is that a cheap DAC with a low-jitter source will be at the same time more detailed and musical (with more accurate, faster tighter bass... but not as boomy!) than a multi-kilobuck DAC fed jittery junk... further you don't need an expensive cable with the Bel Canto USB Link - it comes with one! I haven't heard it but if it isn't so hot, the DH Labs D-75 is fabulous and very reasonably priced. For me, if I were wed to USB output, something like the Bel Canto would give me the flexibility to play around with different DACs buying used and selling until I found one I really liked. I've learned through experience that price isn't everything - there is well thought out cheap stuff that sounds great and there is expensive rubbish... my RA1/RS1 rig has spent most of the time I have owned it hooked up to a DAC (Assemblage DAC-2) I bought used for CA$300... lashed to the RA1 with cables that cost more than any other part of the rig!
Some might argue that John Grado's cans lean more towards euphony than accuracy... either way I do own RS1s and I am a detail nut. I think it depends largely on what sort of music you listen to... the RS1s are tough (read: expensive) to beat on highly produced close-miked multitrack recordings. As always, if you want something that does everything well with no compromise, the price goes up...
I owned an early Benchmark DAC1 and fed it from a half decent transport with the aforementioned DH Labs cable... it was very very good but for me the "musicality" was not there with the RA1 as the amp. Your Mapletree amp would probably fit much better between a DAC1 and RS1s! For me however I like something to sound great in more than one rig so I went up up up the food chain...
The USB in the DAC1 is supposed to be pretty good. You could probably find a USB DAC1 used for under $1k if you are a bit patient, and if it is too harsh, do some cable rolling for the run between the DAC and Mapletree...
You have a great amp/headphone rig... as far as figuring out your source, the best advice I can give you is to be patient slow and methodical, and buy used! The only purchases I have regretted in this hobby have been knee-jerk trips to the shops on a whim to buy something brand new...
Good luck and happy listening!