OK, Ive had time to make sense of it!
<Part 2>
I was going to try and keep this short but it seems my notes, taken over several nights to catch my own changing moods and broaden the musical spectrum, have run away on this one so I’m trying to condense as I go!
Part 1 was the early-ish stage of listening for what the 7963 brought into the mix, Part 2 is an expansion on that coupled to seeing what the “EBs” brought when refitted.
So firstly my summary of what I thought after listening with the 7963s for a substantially longer period on both the HD600 and the K701. One of largest areas of difference here is in the way the valve-swap effects bass. With the 7963s in and using the HD600s, bass is noticeably more tuneful, things like bass guitar scales were far more vivid to follow, being underpinned by the hike in speed and attack. The K701s also managed to define the same tunefulness but lost a little of the very low end, as is their way on most amps.
For head staging the HD600 felt like the music was a little wider than the cans themselves. The K701 were a tad wider still but also displayed a greater homogony to the overall sound-scape at the same time.
The mid range was open and airy, male voices are well expressed with believable tone though sibilant tendencies were slightly exaggerated. The 701 built on this with improved tone. Female voices could get a little shrill which was peeled a layer deeper by the 701.
The high frequencies in general, on good or better recordings, were very clear but a touch too sharp at times. Strangely, overall, the 701 did a better job here, I’ve been used to this being the opposite, except when using the AKClassA.
An easy sense of rhythm is evident, probably due to that mid to upper bass tunefulness and resolution. On the 701 this was a little less “stand-out” clear but there a plenty.
Acoustic instruments were very realistic, you can really hear the different materials used in their design/construction . Small nuances were very audible. This again was favourable on the 701 with a surprisingly fuller tone where the 600 could get a little scratchy at times.
For me the combination was definitely preferable when using the 701, which seemed to lean in the same direction of sound presentation as when driven by the awesome AKClassA, though not pushing them to quite that far up the performance scale.
Switching back to the “EBs” (which was done multiple times) bass instantly reverted to the more typical warm valve sound, almost to the point of being boring by comparison.
Staging was narrower, pulling in more to ear level.
The midrange now seemed muddled, male voices could become either boomy or “stringy” depending on the original perceived depth of tone. Female voices were just....”smaller”.
Generally the HFs now seemed curtailed, unnaturally.
The sense of rhyth was vastly reduced, that open clarity and attach had gone.
Acoustic instruments were darker, the different materials used far less distinguishable. Those lovely nuances were more hidden or glossed over. Overall the EB valves were preferred using 600 rather than 701.
By now you have probably guessed my conclusions. The 7963 are keepers!
They are still not perfect, with that slightly hot HF, but far preferable, on balance to the EBs.
A great find Mike, well done!
(Assuming I've got the pinning right 
)
