Krell HTS 7.1 vs Denon AVR-X4300H

Can this be done:

connect OPPO to KRC3 and HTS7.1 and then to krell KAV500?

I would not do this at all.  You are running the audio signal through two preamps, first a very low resolution KRC3 and then a nice but soft HTS 7.1.  Trying to merge both a 2-channel system and an HT system together with both 2-channel and multi-channel preamps is not a good approach.  Like I said, you really can't have both worlds unless you spend a lot of money.  If you're looking at the really old Krell stuff, I would just get an HTS 7.1 and use it as a 7.1 preamp with the Oppo.  The preamp quality of the HTS 7.1 is much better than the KRC 3.  I know you like the asthetics, but this will not actually improve the sound quality if you have both of this in the signal line.

The MX119 and MX132 are likely going to have similar characteristics to your 2-channel McIntosh system. Mac is typically warm, smooth and laid back sound. Krell is more forward with attack and clarity. Personally, I would go the Krell route if you were putting together a home theater, unless you are after the McIntosh asthetics and sound character.

The MX119 and MX132 are very old. Yes, you can use them as pure analog preamps, but they are only single ended internally. If you are after a pure analog preamp, look into the Parasound P7. It is a much newer device, fully balanced internally, and is only a 7-channel pure analog preamp. There is no digital processing or video capability. It is likely going to have better resolution than either the McIntosh or Krell devices you are currently looking at. If you want to know more about the McIntosh stuff, go over to the audioaficionado.org forum. There’s a McIntosh subforum with a large following. You could post questions there.

However, I will say that there is a "magic" in the Krell Class-A circuits that started with the HTS 7.1 generation which adds something to the sound (the core sound from the Oppo doesn’t have this magic character). The older KRC3 doesn’t have this "magic" character (though I don’t know what would happen if you re-capped / upgraded the KRC3). The soft input stage of the HTS 7.1 will give it somewhat of a laid back sound, but it will keep the clarity and magic of the Krell output stage.