I've been using a POD X3L for about 4 years now and added a Spider Valve MKII 112 to the mix about a year ago. I just plug the live out from the X3L into the "power amp in" on the back of the SV. It's worked well - I don't have to lug around a rack or head plus a 4x12, I can practice silently along with recordings, and I can get just about any tone that I need.
Well, I've been tempted to get an all-tube head, dump the SV, and use the 4CM to just use the effects on the X3L. It's that never ending search for tone while still keeping some versatility. There are great sounding single channel amps out there, but you're kinda stuck live. Yeah, you can turn your volume knob up and down but that only does so much. If you play a variety of tunes in a cover band, that won't cut it. I've been looking at the Bolt BTH100, the Genz Benz El Diablo, and the Randall RT503H. Those all have their pluses and minuses.
Well, I sat down with my X3L cranked through the SV and I realized that it is still a great piece of gear. Plus I don't have to spend a $100-$150 bucks on tubes each year . The stompbox effects do generaly suck, though . That may be what gets me to upgrade to an HD.