+1. The Bifrost is a very detailed, transparent, clean DAC. It's not the most resolving in the world, but it's light years ahead of E17 in terms of resolution and transparency. I often feel bad that I pay attention to all my gear, but endlessly forget about poor Bifrost being there at all. It does its job and gets out of the way so well, it's very easy to forget it exists at all. Which is the point of a DAC. The E17 isn't as highly resolving, isn't as good at separation, and is probably adding a warm color to the sound. With the Bifrost you're hearing your cans and your amp/tubes cleanly for the first time. There's a good chance once you listen more you'll hear more detail you were missing before and realize you were mistaking clarity for thinness. If you still like a more colored sound, it's time for tube rolling that Lyr!
The Bifrost will really let you hear the difference of each tube.
I'll say, though that, to me, stock tubes in Lyr, + Bifrost + HD650 is a great combination and is definitely not lacking in bass. At all. (Some would argue it has too much bass and mid-bass.) The bass in that combo is the same as my carefully tuned 12" Velodyne sub as calibrated in my speaker setup....any more would be true bloat and lose its musicality.) Remember: In sub setup, you shouldn't actually know the sub is there...it should blend with the speakers completely. Same applies to bass integration in well calibrated headphone gear.
Give it time, but if you still like a certain coloration, you can always change tubes or add EQ. Either way the Bifrost should be outresolving and out-timing that E17 by far. The Fiio gear is a great value in its price range (I use an E11 with my IEMs), but the Bifrost is still in a very different class.