I love KT-88s too, and I am not at all surprised the Luxman stuff is turning out to be awesome for you. Since you have the itch already, I will tell you, I ran PrimaLuna integrateds (2 different models) with the Zus (loved them with the Zus), and eventually upgraded the Dialog Premium HP to KT150s from KT88s. The KT150s are a bit better than the 88s in most regards, and worse in none. Bigger stage, even better dynamics, a little better clarity. The 88s are very good...my only reason to mention is that you should try some different stuff if you love the KT88 sound like I do, but want to hear where you could go with different tube topologies. I haven't heard the Luxman stuff, but I have no doubt it's very top notch and so I don't want to push you by suggesting you're leaving a bunch on the table...that would be like telling someone with a really nice 535i that they're really missing out by not getting a 540i. Or a 7 series. Or an M3. They're all great, but different.
I was where you are about 12 months ago--no friends with low-power tubes or local shops that carried anything I could demo. I don't know about your budget, so this may not be an option for you, but I read for months and months and rolled the dice on a couple different used amps, none of which I was disappointed with in the end. I settled on a used pair of parallel 300B monos (Audion Golden Dreams with some upgraded caps) that were highly recommended on this site by some that appear to be serious experts at powering Zu's higher-end stuff. But the two 845 amps I bought are really close and great in their own rights.
The good news is that the used market for 300Bs seems strong, and if for some reason you didn't love it, I suspect you could get what you paid pretty quickly. Might be an experiment worth taking if you can't find a dealer to loan you some. 845s are also interesting...I found them to be nicely between the sound of the KT150s (and KT88s) and the 300Bs.