Using pre-outs on integrated amps can be a mixed bag. Generally, the preamps in integrated amps are "tuned" electrically to mate best with the internal amplifier. It's a crap shoot whether it will mate well with something else.
In effect, most pre-outs are a convenience but the builders don't spend a lot on optimizing compatibility like one has to do with separate preamps.
Anything you do like this would be IMHO a temporary thing to bridge to something else. I would replace the integrated or go separates. Not half of each.