Is it possible to run 2 stereo (integrated in poweramp mode) amps as monoblocks

[Huey2001]

They are. Might it be you forgot to remove the jumpers?

Ok, you are right: did some REW sweeps on the top and bottom posts and indeed they are filtered...

 

[Huey2001]

My pre amp has 2 pre amp outs (MAIN OUT 1 and MAIN OUT 2):
View attachment 491756

Ah ok, nice

 

[EJ3]

Ah ok, nice

Just pointing out that there are a few different Pre Amps (mine is a dedicated Pre Amp [as opposed to an Integrated Amplifier, such as yours]) that have more than one Pre Out.
I have never seen more than one Pre Out on an Integrated Amp (and usually not even 1) so you are most likely correct about that. (when it concerns Integrated Amps).

 

[EJ3]

Have a look at bi-amping audiophile style.
Horizontal: one amp drives the woofers, the other the tweeters.
Vertical: one amp drives the L, the other the R
You need speakers with dual binding post to do so.
Whether you really need that power is another question....

The Well-Tempered Computer

I do an odd sort of mix of Bi-amping. (each speaker [L & R] and each sub {L & R} gets it's own bridged to mono amp).