generally, if the audio interface (is it your POD?) is always the same both for recording and listening, there should be absolutely no difference..
eg if in a different moment you listen again to your recorded tracks using instead the computer soundcard, being the DA converters much worse, the result will be proportional..
also, every time you open a new track in Reaper, is set as a single mono input by default, therefore if you are monitoring your POD in stereo, it'll be recorded in mono in Reaper, so it could be somewhat different, unless you manually select the stereo input in your track..
Well, it turns out the problem was with... my room! I use my monitors in an untreated room that apparently has a deadly bass boost that "embellishes" the otherwise really trebly sound of the pickups of my SG! Changing the settings of some of the switches on the back of the monitors has partially "solved" the problem: now they (the "live" and recorded sounds) both suck! Listening to the recorded track with
PC onboard audio interface + Hi-Fi stereo + Stereo speakers + Vlc
it seems the sound is only just a little worse than when I use
External audio interface + Studio monitors + Reaper
which I also use when playing "live" and recording, but I've noticed that playing "live", when I insert the USB cable from the POD to the PC and even more so when I open Edit, the sound becomes "nicer", don't know why! You're right about the default input settings of the tracks. I thought the default was Stereo (I think the older versions of Reaper defaulted to Stereo) but it's not. Changing from Input1 to Input1+Input2 improves both the "live" and the recorded sounds! Also I have a request: can you record a G chord (3rd fret, 6° string, not the power chord version) with an emulation of the P.A.F (if it has one) on the Variax and the Plexi Brt on the POD, with everything at default settings except Drive at 80%? It would be helpful to have a sonic reference of what they're really supposed to sound like using the POD...