Dreams of amplification

I need to record electric guitars. I decided that I'd use a real amp and a real speaker I just need to make the sound coming out of the speaker real quiet.
So I went ahead and got a road case. It still needs some silicone in the joints. But it went together well.
I also ordered some batting to put inside.
Lastwise I'm getting Redco to put together this plate:

The TRS will go to spade lugs and the XLR's will go to mic cables.
How far will I be able to knock the SPL's down? In my dreams I end up with like 60dB SPL A-weighted with a screaming guitar.
Those are my dreams.