No Idea why I haven't posted on this thread... probably because I just saw it - so a bit of a ghost thread revival. [EDIT: I guess I did reply! Lol, I see my reply above that I somehow didn't see and then wrote the post below.
) Also, I previously mentioned some VC rub... well it turned out that wasn't the case, but I hadn't yet gotten good at tuning the top end and also sorting out proper gains staging with my mesa for 'hairy' tones... so I thought I was experiencing rub on the low end and with breakup... but nope... all good. I have 4x of these speakers, and generally run them as a 2x12 for best results.
I am a diehard fan of the D123's (pictured). I have a fairly complex rig that gets me to where I need to get though. I'll share some examples since without examples.. well we all know... words can't sing like good tone can!
Anyway, the EQ tilt for clean tone lovers, and the overall extended frequency response while still retaining the classic JBL punch on the low end is what drew me to these speakers. In a semi-open back or fully open back BB or Pine/BB combo cab, they present a 3D sound that is slightly diffuse in the upper midrange. I think this is a positive, because the less directional nature of the amp in this range allows the volume to be higher without the shrill nature of the upper midrange getting out of control - meanwhile the true highs are not attenuated and this is what is often missing in clean tone.
Now obviously its going to reveal your technique, and you will have to dial a rig in to suit both your playing, picking style, etc... Also you will hear more string noise so you will have to be cleaner. I use a TC1128 Analog Rack Mount EQ with Digital/MIDI control to tame the highs before it hits my McIntosh MC250.
I also go into a Mesa 20/20 after my Sarno Pre (RCA Blackplate 7025). The Mesa is loaded down by a Suhr Reactive Load box which I connect to a loop on my RJM Switcher. I basically dial in control of power tube saturation with those sweet chimey EL84's which gives the Jerry bite when digging in but without getting thin or harsh.
In general, distortion tones will need work with this kind of speaker. It will help to take a look at Celestion curves, or classic british style speaker response curves and dial those in post distortion pedal(s). This is another reason I use the TC1128. I can MIDI program in the 1128 to switch over to my response curve of choice when I hit my distortion pedals and really dial in the EQ curve. In general, all that glorious top end that sounds good with clean or hair of the dog tones is nothing but frizzle frazzle with heavier distortion, so we must tame everything above 5K and also dial in some reductions in the midrange and upper midrange (hence studying the response curves of classic speakers). The D123 ends up being a clean and transparent platform that takes well to EQ and sonic manipulation... but requires work for heavier tones... but to me, it is worth it for the heavenly clean tones that are anything but thin or anemic.