Hi guys,
First of all, thanks A LOT for your feedback to both of you!!
Sorry if it took me a few days to reply but this is still a bit dense to me.
I'm trying to understand and digest all this info and do my homework so please bear with me

The original Cabsim circuit was designed with J201 and BC550/BC560s. I can't find these at Mouser, they're way obsolete. I can't find cheap J201 TO92 subs so I'll probably get some MMBFJ201 instead and use a small adapter board for them. Can I sub the BC550/BC560s with BC546/BC556s? I found some with extensions BC546CTA/BC556ATA but I don't know if they have the same gain and/or are compatible to be used in the circuit.
Now on to the balanced output circuit. I read the article by Bill Whitlock, that I found very interesting (thanks for the link). I understand (well I try) the concept looking at the typical unbalanced and balanced outputs schematics he shows. But then I get lost at some things you mention
Sorry for the language barrier...
For an opamp you need to split the rail, meaning bias the non-inverting to 4.5V with a 9V supply. It's a common technique in pedals.
You mean like this?

You may have to increase R1 to about 100k, in consideration for the 4.7k resistance at the output of the cab simulator.
You mean the resistor between the opamp +input (pin 3) and GND, right?
For a more complete approach, check attached pic
Sorry if I'm slow, I'm trying my best to follow...
The pic says "output of the cabsim goes into opamp pin3 after a 10k resistor between the output and GND, and 9v to pin 4 and GND to pin 8"?
So the Elliott Sound Website circuit is ok? I'm asking because he says it's cheating and after reading the Jensen paper I don't know...wouldn't this be the proper way to go?

Thanks again for your time and help
Cheers
Sono