AMR DPP-777 DAC tubes

The best thing you could do is remove the valves, get a few cotton balls, put a little WD40 on the cotton balls and plunge the valve pins a few times. Wipe them off real good. Don't get it on the glass.  If they are NOS Tellies for sure and a few others. Wiping will remove the crappy printing..

Don't wipe off the pins, just plunge them a few times in DRY cotton balls..

Re install the valves..

Can you take a pic of the tube and post it? Some tubes are easily recognizable even without the name on the glass.

A DAC doesn't use much current so the tubes can still be good after three years. But knowing what the stock tubes are can tell us if performance can be improved.

6N11 is the chinese equivalent to e88cc, so plenty of alternatives, but check first with manufacturer what would they suggest.

I replaced the tubes in my DP-777 a year and a half ago simply due to the fact that the stock tubes had seen a lot of use. Also, the DP-777 (it is not "DPP") is weird in that on standby mode, the unit powers up when there is a power outage. Here in Central Ohio power outages are common, often for less than a minute, but then many days and even weeks would go by where I would remember the effect of outages on my DP-777-that it was powered up. Quite annoying. 
But on to your question. I heard very little if any change with a change of tubes. I bought new versions of the stock tubes, nothing particularly exotic or expensive. I do not think the tubes in the AMR DP-777 are implemented in such a manner as to have very much effect on the sound character. I like my DP-777, enough to hang onto it though I am presently using a cottage industry hand-built, non-oversampling, R2R chip based DAC-the SW1X DAC III Balanced. Unlike the AMR, the five tubes in the SW1X are implemented in such a way as to affect the sound.