Dave,
DR9's are not balanced, they are designed to operate in stereo & sound
considerably more open, real, as well as transparent than they do in bridged
mono even though they have the option to do so.
I bought my DR9's as a pair, and as much as I hated to part with one, after experimenting
with them in stereo/mono it was a no-brainer that they are a superior sounding
unit in stereo.
In mono, they sound a little flat, a little lifeless & not as involving.
Funny thing is even though they have twice the power bridged, the amp does
not have the same depth or the slam that it has in stereo due to the music
sounding veiled.
I was told by two separate shop owners who sold DR9's that this was the case, but didn't buy it until
I experimented for myself, and also found that the better the speaker the more
obvious it became (Technic SB-100 - Roger Sound lab Elans - Focal Electra 926 -
then Meadowlark Heron Hot Rods & now 946 Focal Electra's).
That said, after 27 years I recon the caps are getting kind of dry & a referb
can't hurt.