1) congratulations on what you are trying to achieve

2) it *can* be done

3) that said, "the devil lies in the details".
As an example,let´s check the first gain stage, which tries to emulate the first gain stage in OR15:
* first triode has gain about 40 to 50X
Your first iteration also used 40X gain ... problem is that OR15 first triode is fed some 200/250V DC so it can put out some 70-80VAC without clipping , while an Op amp will clip above some 9V RMS.
So exact same guitar which is handled flawlessly by the Tube will horribly clip the Op Amp.
Of course, lowering gain (your second iteration was about 15X) will improve the outcome ... but as you see now we are getting away from original schematic operation so we´ll have to account for that somehow.
As said above, it can be done ... but not "literally", we´ll have to compensate, and that at every stage.
* clipping diodes clip, but:
a) they clip symmetrical (tubes do not)
b) they start clipping at 0.7V peak while Op Amp would only clip (on its own) at some 12V peak
This alone will also change your design even more.
Just as an idea: put a reverse Zener diode in series with each conventional one; say a 9V one on top and a 12V one on bottom.
Now signal will clip on +9.7V and -12.7V bottom.
Also use widely different resistors in series with each: low value on the 12V side so it sharply clips, a higher value one on the 9V side so it loses gain but does not actually *clip* but sinewave gets rounded (like a tube reaching the saturation area).
Now you have a stage which mimics triode clipping way better.
Apply same kind of tweaking and correction to
all tube stages and you´ll get quite closer to the original full tube one.
Just warning that power stages can get REAL complex ... and I
mean it.
But congratulations again and keep experimenting
