I can't speak for the tube buffers since I've never used them. In any event, I would think a tube buffer would be wasted on the subwoofer channel.

Almost all of my system is tube based, and I have never found that tubes need any more of a warm up period than perhaps 5 minutes at the most. Tube power amplifiers actually reach optimum operating temperature faster than solid state power amplifiers because of the bias drift inherent in solid state designs as the heat sink temperature stabilizes. The bigger the heatsinks, the longer the stabilization period.

I would not leave tube gear on at all times. The constant emission from the cathodes shortens tube longevity more than any turn-on power surges might. Some tube gear has surge limiting in any event, so this is not an issue. Ironically, the older tube designs which used tube voltage rectification had built-in surge limiting by virtue of the fact that the rectifier tube required a finite amount of time to warm up before it passed current.