Help Me Connect Two Preamps to One Power Amp

[exztinct01]

Above is ESP Project 115.
I want my guitar amp (P27 Preamp and P19 Power Amp) to have an MP3 Input but since my player (smartphone) has low gain, I decided to add a preamp for the mp3 player inside the guitar amp.
Now, can I please ask for guidance what kind of circuit should I put in between the mp3 preamp to power amp, and guitar preamp to power amp. I think it might be better if it's illustrated but a clear explanation and guide is enough.

Also, I don't need that many resistors and the relay for the preamp since I'll only be using it for the mp3 player so I need it simplified too.

[Enzo]

The first part is simple, they have four inputs and a switch, and you want one.  So wire the one jack directly to the BL wire.  The eliminate the switch and other three jacks.

The thing below U2a would not be a relay, it looks more to me like a 4 section DIP switch.  Only three sections are used.  By turning the individual switches on and off the three R7 resistors are used to make different combinations.  The resulting total R7 with R8 sets the gain of the stage.  In other words it is a volume control.  As I look at it, it seems the combinations got with the switches being of or on range from about 2.7k up to 10k and also open.   You can experiment with those three values and others from U2a pin 2 to ground to see what results in the gain you prefer.  Or you could mount a 10k or more trimmer pot there.

[exztinct01]

so I can make R6 and R8 fixed with those values? And then add the trimmer to sub the 3 resistors.

How bout that section where the outputs of the two preamp and the input of the power amp meet?
What values of resistors are recommended there and what kind of connections?