Well, I don't think anyone has answered your question, which is whether you could use the regular preamp connected to the balanced inputs on the amp, and a another preamp connected to the unbalanced inputs on the amp. If there is a selector switch to pick inputs then this is probably ok, go ahead and try it in my opinion. If not I'm not sure you may want to see if anyone else answers this.
Now, as for an outboard phono preamp in the tape loop or something, I tried this but found that the gain on a number of units was too low. (However some units have adjustable gain so make sure you check on this.) You switch from cd to tape (phono) and and then have to turn everything all the way up.
For me the better choice was to get a stand-alone preamp that had a phono stage and put that into the tape or aux input, so you can equalize volume easier. You will note that phono preamps are very expensive and I found a nice preamp with a phono stage for the same price.
Now, as for an outboard phono preamp in the tape loop or something, I tried this but found that the gain on a number of units was too low. (However some units have adjustable gain so make sure you check on this.) You switch from cd to tape (phono) and and then have to turn everything all the way up.
For me the better choice was to get a stand-alone preamp that had a phono stage and put that into the tape or aux input, so you can equalize volume easier. You will note that phono preamps are very expensive and I found a nice preamp with a phono stage for the same price.