So I did a couple of mods to the 5150 III recently that really make the amp quite a bit better. First off, I have yet to hear anyone that likes the stock blue channel on this amp, which is an absolute shame because all the other 5150 III heads have an amazing blue channel. It's come to light more and more over the past year or so that simply snipping out a small capacitor fixes this.
This is the easiest mod you can make to this amp and it totally kills. Just snip out capacitor 137 and the blue channel comes to life, making it just as great as the blue channel on the other 5150 III amps.
Now for the second level of experimentation. Now that the blue channel rocks, let's make some changes to the red channel. This will be easier, as it's just tube preference.
I went into this morning wanting to mess with preamp tubes to make two changes to the red channel of the 50w EL34 amp:
1) make the gain knob more of a usable sweep, and
2) get rid of the squeaky midrange the red channel can have.
I wound up with this configuration that accomplished both:
V1: Mesa SPAX7-A (a decently high-end 12AX7)
Red channel specific tubes:
V2: JJ12AU7 (much lower gain)
V3: Sovtek 12AX7LPS
V4: Mesa 12AX7
This worked! I highly suggest replacing V1 and V4 with higher-quality tubes than the amp ships with. The 12AU7 reduces the gain of the red channel, making the gain knob useable throughout instead of topping off at 10 o'clock. The Sovtek LPS gets rid of the mid honk, making the amp sound much more balanced. If I had any spare Tung Sols lying around they would've gone in V1, V4, and V5, but the Mesa tubes will do just fine for now. All-in-all I recommend experimenting with the tubes in this amp and had success with this today, for sure.