That's true. All I remember is that I still heard hum at one point after I installed Q11, but not anymore - the amp seems to be working fine. I'm betting it was Q11 that was causing the issue.
Everything's put back together and we're testing all the knobs and buttons.
If the issue comes back, I'll know it's an intermittent issue caused by something else, and I'll post another thread on the forums.

You've all been a great help, and you saved my friend and I loads of money. And I learned a thing or two in the process. We're both real grateful - thanks again everyone!

Yeah, no more hum. Just normal amp hum. Had the leads on the speaker outputs and watched the voltage drop from 10 mV with the guitar plugged in, to -400 mV when it got unplugged.

Measured the voltage on the output again and it read -400 mV. This is strange because right after I installed the new transistor, it was still delivering the same hum to the speakers. Output voltage measures 10 mV with a guitar plugged into the amp. I plugged the speakers in and played the guitar a bit and got sound from the amp.
Safe to say that it's fixed? Or should I check anything else before putting everything back together?

These were measured with the negative lead to the cathode, and positive lead to the anode:

D21: 13.7 kilohms
D22: 13.7 kilohms
D23: 13.7 kilohms
D24: 12.85 kilohms

Couldn't measure anything with the leads flipped.

I don't have a diode test setting, so I measured resistance across it. It was 13.5 kilohms, no measurement the other way.

Alright, I understand now. I edited my last post, so the measurements should be correct this time.

Transistor readings are each point to chassis ground:

Q9
E: 1.47 V; C: 42 V; B: 3 mV (essentially nothing)
Q10
E: 1.9 V; C: 42 V; B: 5 mV (essentially nothing)
Q11
E: 42 V; C: -43 V; B: 42 V

Diode measurements:

D21: 1.12 V
D22: 1.16 V
D23: .29 V
D24: 1.47 V

New 2SA1016 came in the mail and I soldered it to the board and reconnected the speakers. Still getting buzzing. Still measuring 42 V from collector of the new Q11 to ground.

Sorry, misunderstood when Roly asked me to measure the transistors.

Q9 from collector to chassis ground: 440 mV
Q10 from collector to chassis ground: sits at 7 mV for the most part, but fluctuates intermittently to 0 mV and 24 mV
Q11 from collector to chassis ground: 42 V

Also measured R87 as DrGonz78 suggested. Multimeter read 42 V, measuring both sides of the resistor to chassis ground.

Voltage across:

Q9: 40.3 V
Q10: 41.4 V
Q11: 82.4 V

I tried measuring the voltage across R87 for about 15 minutes. Every now and then I get a reading on my multimeter, but I don't know if it's reading the voltage across the resistor. It'll read around 12 mV intermittently. 90% of the time, the multimeter will read zero. I attached some pictures here: http://imgur.com/a/DmXuw#0
The leads are definitely making contact. I don't understand it either.

For R89, 90, 102, 104, and 106, the values changed when I flipped the leads. The values did not change when I did this for R87 and 88. Here's a link to some pictures I took, as well as the values I recorded from my multimeter, measuring both resistance and voltage: http://imgur.com/a/iN7Zz#0
"+ top(left) / - bottom(right)" means the positive lead of the multimeter made contact with the top(left) lead of the resistor, with the negative lead making contact with the opposite end as determined by the pictures of the top face of the board. The same goes for the opposite column.

Thanks for the help so far.
Here's a link to some photos I took: http://imgur.com/a/WFPFq#0

There's an offset voltage on the output of -1.19 V. The diodes pass current just fine. I checked the solder joints for each capacitor, and they seemed to be making contact just fine, but I noticed a possible short on C75 - you'll see it in the photos. I also measured AC/DC voltage through each capacitor:

42 VDC / 92 VAC across both C71 and C72
15.6 VDC / 32 VAC across C75
16.3 VDC / 35.7 VAC across C76

I hope this information is useful. What would be my next course of action here?

Edit: I was also unaware that there should only be DC flowing through the caps. I provided the AC readings in case they proved useful, but they're most likely not.

I took a close look at the speaker and watched it kick back and stay there when I powered the amp on, so it's definitely DC at the output. Someone told me it could be caps 71/72 or 75/76 that need to be replaced, or D53-56 might be shorted. Since I know that the DC is the issue, is there any other component in the schematic besides those that might be the culprit? I'll have to get my hands on a multimeter to check the offset voltage and to check for shorts in the diodes, but would it be safe to say that replacing all four caps would solve the issue?

My buddy has a Fender Frontman 212R solid-state amp that he bought used from Guitar Center, and it's been performing fine for almost a year now. He plugged it in a couple days ago and it produces this 120 Hz buzzing, even without an instrument or cable plugged into the amp, and even with all knobs set to 1. My first thought was that it was a grounding issue, but we checked out the inside and everything's grounded properly, and nothing's wrong with the IEC cable. I did a scan over the board to check for any damaged components and made sure everything was plugged in properly. After looking around online, some people are saying that there's either DC on the output of the amp due to a bad output transistor, or it could be caused by a bad filter capacitor. I haven't taken the cover off the front of the enclosure to see whether or not the speakers were moving, but I can feel them vibrate when I touch the backs of the speaker cones, so I don't think there's DC signal getting sent to the speakers. I'm also not the best when it comes to electronics, especially amps, so this one's really got me stumped.
I should also add that the buzzing doesn't change when any of the pots on the amp are turned and regardless of whether or not a cable or instrument is plugged into the amp.
If anyone could provide some insight as to what might be the cause of the hum and/or any possible way to fix the amp, that would be appreciated.
Here's a link to the schematic of the amp:
http://www.amparchives.com/Amp%20Archives/Fender/Schematics/Fender%20Frontman%20212R%20Schematic.pdf