When the SLM era cabs were out, I was playing a PR
series 810 - no comparison available there.
Since those days I have played a wide variety of Ampeg 810 cabs from different eras (in back lines) and the Heritage (that I currently own) beats them all hands down.
To MY ears - and this is only MY opinion- the articulation of both the highs and the mids is much greater than the earlier cabs that I have played. Lows are still there, but seem to be a tad more "pronounced".
I can't say if there was any additional engineering that went into the Heritage 810 - there were upgraded parts in both the Heritage CL (wiring, tubes) and the 810 (speakers, hardware and wiring) but whatever the heck they did - I am a fan.
Jimmy M recently grabbed a rig himself and he has compared it favorably to the Blue Line era. I never had the opportunity to play a Blue Line, So I am more than willing to bow to his expertise. All I know for certain - is that the Heritage is the finest Amp/Cab combination that I have ever played.
I currently have the last rig that I will ever buy.