The amplification section in the HM-901 it's "any of the cards", so when you use the line out you get the signal before the cards and gets a line level output (like a cd player) to use with external amplification like a full size stereo system. Of course you can use any amp that you like, and the balanced amp that you mention looks quite good, but you have to keep in mind that you need fix all the cables needed and that there's one more box with another battery to carry on. Also you should keep in mind, when you evaluate the possible advantages of the raysamuels', that the price tag of the raysamuel's includes the extra box and all the interconectors and volume control section that the HM-901 cards doesn't need or are already included in the player. It would be useful to know exactly the components and specs to evaluate if it's any better than the hifiman's cards and don't get fooled by the price tag.
Obviously our friend it's looking for an improvement in SQ from what the HM901 offer with the balanced card. I agree that the SQ and the convenience from the HiFiMan's cards are hard to beat but you can not be sure (at least about SQ). I'm just saying you can try as many options as you patience and wallet can handle but if you ask my opinion, yes the raysamuels' looks quite good but better than the balanced card? I don't know but you can not tell just for the price tag. You need a spec sheet to compare and even then the only real test is A/B. Anyway knowing the spec can give you an idea of its worth the effort.
