AMR DPP-777 DAC tubes

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. 

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. 

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.