BRM,
Let's compartmentalize. Can you perform a sweep limited from let's say 15Hz - 350 Hz, without any smoothing, while making sure you are able to see the peaks and nulls? You'll have to play with the zoom feature on REW a bit. I take it that the mic is at the listening position, and your speakers have been set for best staging and imaging.
Extend the waterfall "time" to like 400-500 msec for the 15-350Hz bandwidth so you can see all the tails decay into the noise floor.
Bass is 30% of overall system performance. Once we improve that, you should be able to hear improvements in staging, tonality and so on even up into the midrange/treble.
The RT60/30 plots are hard to interpret in small room acoustics as they were meant for large room acoustics. Nevertheless, you have a "dry" sounding room. Lots of arguments abound but 0.3 is nice, greater than or equal to 0.5 is a bit too lively/bright sounding, and 0.1 and 0.2 is dry. It's enormously difficult to get 0.2 at 20 Hz-100 Hz levels! But it is not necessary to have low RT's in the bass region. Less than 0.5 is good. What RT level is noteworthy depends on the
frequency range we are talking about. In any case you don't want wildly different RT numbers at 300 Hz versus 3khz for example even if your FR is flat. Redo your measurement on REW so that you are concentrating on 15 Hz to 1khz, and you'll see what I mean.
An excellent read is this:
http://www.hedbackdesignedacoustics.com/files/QuickSiteImages/AMS_for_Stereo_List._Rms.pdfTry hard to digest the above link. You will reap the rewards in doing so
Best,
Anand.