2 stereo amps in a mono-block config = 2 x power?

Pre amp "out" signals are usually 2 volts or less. Your power output of the amp would be the rating of the amp. Are you talking about 2 separate compontents when you say the "manufacturer offers"?
It sounds like you want to connect the preamp "out" of your integrated to the preamp "in" on the same unit? Maybe I missed something.

I can't visualize your set up--do you just have the integrated with two stereo amps built in, or do you have an integrated with a stereo amp built in and a separate, standalone power amp? In either event, going from two stereo amps to two mono amps with twice the power is not always possible--stereo amps have to be "bridgeable" to do that, and its something that most amp manufacturers don't build in...

My setup is: I own a stereo integrated amp rated at 90wpc into 8 ohms. My speakers require more power than my integrated can adequately deliver at higher volumes. My intent was to buy a new separate power amp to connect to the pre-outs of my integrated so I could at least utilize the pre-amp section of the integrated, saving myself the expense of replacing it with a separate pre-amp. I asked the distributor of my integrated for pre-out specs so I could properly match it to a new separate power amp (I've read that the power amp input impedance should be at least 10k times that of the pre-amp output impedance). But instead of providing the requested specs, he suggested I purchase a new stereo power amp from him. One which is an exact duplicate as the power amp section in my integrated. Then use the two stereo amps (one stereo integrated and one stereo power amp) in a monoblock configuration: one channel from the stereo integrated amp dedicated to one speaker; and one channel from the separate stereo power amp dedicated to the other speaker - so each channel is driven by a different amp. He suggested that would provide each channel with 180 watts of power into 8 ohms, since only one channel was being loaded per amp. Is this clearer?

No, you'll still get only 90w per channel with the setup you describe, though having an extra power supply may make things sound a bit better. Not using one of the channels doesn't double the power rating of the remaining channel.

If you want more power you'll have to replace the integrated.

If the integrated's internal amp's two channels can be bridged into one channel driven by one of the integrated's preamp channels, then yes, you can benefit from more power as a single channel amp here. I highly doubt the integrated has such a feature....but you never know. Maybe there are jumpers in the unit to provide this.

If the above is possible, you have one channel down, one to go. The problem now is to find another amp with exactly the same sonics as the newly bridged amp in the integrated. This is even less likely than the bridging option to begin with. The last thing you want to do is drive the speakers with "different" amps. And yes, the one IC to the external amp would certainly botch up the match between the two channels even if the two amps were the "same". And locking yourself into an amp that matches the one in the integrated is simply too limiting. This is a goofy proposition all the way.

I would consider any number of other possibilities before I went through the pain of what you have proposed:

1: If you want to upgrade a little at a time, find an amp that will drive your speakers to the musicality that you desire. I suspect that the integrated's preamp section will drive all but the lowest sensitive amps (typically matched with high output tube preamps) just fine.

2: Ultimately determine which is more important to you, the speaker or the integrated, and look to find a more suitable mate to the piece you decide to keep.

3: It looks like you are using a subwoofer. Is this not buying you a lot of headroom for the amp in the integrated to give the presentation more dynamics, control and slam? If you are driving the main speakers full range, perhaps you can crossover the speakers with a high pass filter as the sub is handling the bottom 2 octaves or so. This would free up a lot for the amp to more easily drive the the remaining frequencies in the main speakers.

4: Investigate other reasons why you do not have the dynamics and/or control in the sound that you anticipate is lacking now. As my sources have become more refined and resolving, I find that I play the music lower in volume.

Hope this gives you some ideas to consider.

John