I shall contribute my impression after a month of using latest CLAS db and RX3b.
I haven't receive balance to balance ic between clas and rx3 so all the below impression all in single ended config. Source is ip4 usb to CLAS using the stock usb IC that comes with the CLAS.
One word: Impressive.
I tried a few high end portable amp demo set at one of the shop here in singapore none impress me. I own pico dac/amp before and that doesn't impress me either so I ditch portable setup all together and go for desktop setup. Now using MDAC with lyr with ecc188 tube. Till I come to know there's balance portable setup I straight jump with this setup after short audit.
I found this amp need warmup, sound better if you feel the amp is warm. Cold start sound harsh.
First I use with HD600 defoam, stock cable: first I notice it's very dynamic yet full bodied, high are airy, Sound with a lot of space, layer, clear, distinguish. of course this headphone doesn't give you speaker like staging but I can still feel the space and airiness. Bass is deep and tight, I have the bass nob turn close to max, sound fuller. Plenty of volume left at high gain setting. Enjoy listen to this setup a lot.
second I try put on LCD2 with mandrof cable that I made: similar to the above. but mid sound less forward which I like. Better layering and staging as compare to HD600. Bass is little less then hd600 still I consider great. As with HD600, there's plenty of volume. In short good paring with lcd2 r2.
I got Jh16 as well in single ended mode as balance cable hasn't arrive. Ok, even at low gain, the hiss is there, for sure. for single ended user, there's a easy solution, get a inline attenuator and turn down the attenuator nob from max till you can't hear any hiss. then up the volume of the RX3, this method you still get plenty of volume, if not change to mid or high gain. Somehow I feel for JH16 this rx3 is really overkill, lolz. I will wait for balance setup to pass commend on this pair. Not sure if in balance mode will the noise get better, if not, balance user may find it a problem as it not easy to made balance attenuator (can't even find the connector)
The only thing I not happy about this RX3 is the volume imbalance issue at low setting. At cold it's worse, after warm up, gets better, but I still paranoid about the imbalance thing as it made me feels it always imbalance even after warm up or crack up the volume its get to the point of indistinguishable.
I hope there's a better volume nob with tighter spec, this amp deserve much better volume attenuator.