There are some, and apparently you are one as was I, that think the stock tubes in the Cayin make it sound a bit hard and brittle.
I have listened to numerous amps, and the Cayin A88T however, is not in the hard and brittle catagory at all in my opinion and for my preferences, with the possible exception to some as to how it sounds with the stock tubes.
However, changing the sonic signature significatnly toward the sound you want with the Cayin A88-T, relative to the 'lively/stark' stock tubes, is an easy task with tube rolling.
Knowing what you think of the sound with the stock tubes, gives me a very good idea of what direction you want to go and what will work, as I have done alot of tube rolling and experimenting along those lines myself with the Cayin A-88T, prefering a strong musical midrange presentation and hating anything that smacks of hard or brital sound.
I recommend you buy a quad of SED KT88s (my preference) (or SED 6550), a pair of RCA round plate 6SN7s, and a pair of Raytheon 6SL7s for a wonderfuly warm and musical, open and non-harsh presentation which exceeds by my ears, the musical capabilities of most amplifiers around today.
Stay away from Tung Sol 6550s, ElectroHarmonix, and Genalex Lion power tube offerings, unless you want to bring in a level of extension that you will probably call hard and brittle.
Enjoy your Cayin A88-T. You have a wonderful amplifier.