AMR DPP-777 DAC tubes

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. 

Thanks for the very helpful information. It looks like there's no real advantage to upgrading the tubes in this DAC, so I'll just let these live out their useful lives in peace and go from there. I'm not really very technically sophisticated about the language of electronics, but I am intrigued about R2R DACS based on what I've read. Would be an interesting experiment to try one (maybe not the SW1X which weighs in at about 6K, way beyond my budget) and see if it's significantly different from the AMR. 

maybe not the SW1X which weighs in at about 6K,

6K for a DAC II Special which is indeed quite special. 13K for a DAC III Balanced which is superior to the former but only by a fraction. More apples and oranges than higher on the pyramid of musical truth. Just my humble. 

Update: after some months of mulling this over, I decided it would be an interesting exercise to see what the tubes in my DP-777 actually are before further considering replacing them. As I've stated earlier, I'm no kind of electrical engineer, so just the process of getting into the works to examine the tubes turns out to be rather daunting. There are clear plastic plates on either side of the top, under which some of the workings (including the tubes) are visible, but I don't think these plates are easily removable, so It looks as if you have to remove the back of the unit to get inside, despite the dire warnings about the risk of electric shock. I got as far as removing the thin back plate, but underneath that is a far more substantial metal plate that anchors all the input and output receptacles, all held in place by a number of hex-drive bolts. At that point my courage failed me, as I figured that all the receptacles are wired in place and it's not something I feel comfortable disturbing. For any of you who have replaced the tubes in this DAC, is this the procedure you used? Is there a less intrusive way to get at these tubes? The prospect of having to send it to iFi/AMR (assuming they even still service these DACs) for this is unappealing. Thanks.

It took me a while to figure it out with some trial and error and head scratching but the way you ended up trying is not it. It has been some time but IIRC I removed the sides and slid the circuit board out. Contact the importer for AMR/iFi and I am sure they will help you out. Any qualified tech can figure it out if I can. They see a lot of designs including the weird case configuration of the AMR.