Didn't Viren say he could do this for you?
The power amp section on HK amps is normally on a separate board in the middle of the amp. It should be a very simple matter to locate an entry point for the preamp signal, and drill two holes in the back of the chassis for the RCA jacks or a single one for an entry cable. I recommend the latter, with adequate strain relief. Can you remove the cover and take some pictures? I could help you identify the input points. a schematic would also help, but I don't have it for your amp.
TP generally means Test Point. I'm afraid my connection is very basic and downloading it may become an issue right now, but I could try in a day or two.
The cable is usually a single shielded cable, but I don't remember the connector, on the few HK manuals I have it is displayed as a 4-pin cable. Generally powering up the amp and disconnecting the cable, and using a signal generator will quickly point out the input of the amp. The input is normally labelled, so yes, LGR should be the right marking. IIC they are no hard soldered, but I don't know how it is on your amp.
You cannot connect two preamps unless they are in series or switched. This will mean an extra switch or jumpers like the higher end HKs. Series is useless, as it creates even more distortion by chaining the pres together. If you need to, then you'll need a switch on the back to disconnect and reconnect the HK pre. It should work out fine for you, but frankly I would rather you commit to one or the other. The soldering is simple enough, so if you feel compelled to 'go back' you could always do so.
I hope you are aware that this considerably affects the resale value of the HK.