Hey, it's no big deal.
All 1x15 Mesa cabs, no matter if it's the RR, Powerhouse, or Diesel are the often tried and true TL606 speaker cabinet design - 1 of the best and most efficient configs out there.
I loved my 400+. There is something to be said for a matching head and cabinet rig. I ran my 400+ with a Mesa 2x10 & 2x15. It was a phenominal setup. After I didn't play large venues anymore the setup became to cumbersome. So I sold the 2x15 and went w/a 1x15 & sold the 400+ and got a Mesa M2K.
After reviewing the multiple threads here on TB about 2 cabinet effiency: running 2 identical cabs vs. 2 cabs w/ different sized drivers, I ended up finding another used RR 1x15 and went that route. The difference in sound between my old 2x10 + 1x15 vs. 2-1x15s was startlingly apparent.
So I'll save you the time & trouble, start looking for another Mesa 1x15.
72 Les Paul Triumph, 76 Fender P, 76 Fender Jazz
Marshall JCM 800 SuperBass MK II w/4x12, Mesa Boogie M2000 w/2-1x15s