AFAIK, I think both (T-muso and Govibe) of them are from Burr Brown PCM270X (T-muso is PCM2704). I paired T-Muso and E6 with my 668B, and it's a lot better now. Lesser sibilant and harsh highs (but it's still bad with certain songs for me), more transparency, instrument separation and better controlled bass.

But one thing is that you need to get your own custom casing for T-muso (a simple Eclipse candy box, but you might have to bent the OScon capacitors if you're ordering the OScon caps, or a card holder with anti-static sponge would do nicely), whereas Govibe has one already.

Audio Technica and its ATH-AD300. Might be less bassy, but it's several times more comfy than 'Lux and has a smooth SQ. And that happens to be my next route after testing it in Jaben

My my, ain't that pretty a 661. Makes me wanna get one now

Hmm, FiiO might be a good pairing with Superlux 'phones. But then again, I can't be so sure, as I only own an E6 and it's paired nicely with my neutral 668B. Gives it a nice layer of warm SQ.