I'd like to hear them!
You could get thee another mc240, have a pair of 80 wpc mono-blocks.
Play very dynamic music loud with your existing one, then play with two, decide: keep/use the pair, or original single was enough all along.
You will know if more power is beneficial (and live with the additional tube heat) and keep the 6L6 sound.
IF you go mc 275, you move to KT88 sound (or 6550's), more heat, and the potential for a 150 wpc mono pair of mc 275s in the future if change gets in the air again.
note: I just read that the stereo 16 ohm taps are not available when bridging these models into mono blocks. those XRT20 speakers say they are 8 ohm, not an issue, I just mention it as my vintage speakers are 16 ohm.