Since forever, guitar players have been boosting their tube amps to get more gain, better tone, a volume boost, etc. It's often a lost case for a basement player to find a way through all those pedals and methods. I'll do my best today to give you some hints to find what you need.
There's a ton of options on the market right now, but sadly, I'll stick to GUPtech (and Jonny Rock Gear) products since I can control and understand those. Feel free to scroll forever on forums and Facebook to find someone who's telling you exactly what you'd want to hear, I won't do that here.
First : What's the need?
Why would you want to boost that amp? You've heard you need to? You want more "gain"? You want to remove "flubby" low-end? Tighten your tone? What do all those things means?
If you want to add a bit of gain to a tube amp, there's a lot of solutions for you. You can use a Boost pedal, you can use an overdrive, you can scrap that amp and use a distortion pedal (please don't scrap working amps...).
1 - The whole Overdrive Overhype
So the first option will be the classic "Tube Screamer", the name is telling you what you want to hear.
The GUPtech version, the Honker Hot Rod, or the Jonny Rock Gear one, the Dude Screamer, will help. 
What will an overdrive do to your tone? The typical use would be with drive near zero, a lot of volume, and tone to taste. Pushing volume into a gainy preamp will add a nice slice of gain while removing a bit of the low-end, turning your tone into a piercing "I can hear myself in the mix" great tone. This trick is good with Marshall amps, Mesa Boogie amps, German amps, anything that will crunch.
Of course, there's plenty of other overdrives to talk about, but I'll limit myself to the Tube Screamer here.
2 - The "clean boost" approach
Some amps already have a lot of gain at the turn of the "drive" potentiometer. But still, you may feel like it lacks something. What is it? Is it too much bass, even with the presence and High EQ pushed, sometimes it is a feeling of not having much definition. From there you enter the whole world of the "clean boost" in front of the amp approach.
What does a clean boost do? It takes your guitar signal, pushes it stronger, more volume, and kicks the front end of your amp with it. What happens? If the amp is already in the gainy territory, you'll get more gain, but you'll also notice that for most amp, you'll get more definition, less low-end boomy tone, a nice mid hump maybe?
How to know what you need if you want to use that approach is mainly what kind of instrument and amp you're using, and what kind of music you want to achieve.
The All-Around boost :
Peeps boost is a first step, one-knob, all frequencies are pushed equally. You'll get a fat high gain tone. Perfect for Hard Rock, Metal and pretty much anything violent.
Le Chiou is the next step, full EQ, control over your frequencies with tweakable controls. You have too much high? Shut those down. Want some mids? Here they are! You'll notice something "new", the "Tight" knob. But what's that? Acting like a low cut filter, the Tight knob, as you raise it, will remove almost all low frequencies, starting from super-low, to almost mid-ish. Turning your amp into a super tight and punchy machine. We'll see the use of that in a lot of music genres, 8-strings stuff, low-tuning, etc.
Still not convinced? Maybe a 3-in-1 GrindR pedal is what you need? In "B" mode, you'll get a LOT of volume, but without any low frequency, just like using the Tight knob of the Chiou. That means super tight tone, especially great with low tuning stuff. Need a bit more versatility? Kick the P mode and get control over the Bass and Treble, and still get a LOT of volume. Did I say A LOT. A LOT. A LOT. Why am I screaming again?
3 - The Crunchy boost option?
If your amp lacks a bit of "crunch", maybe you want to have a boost with a bit of grit. Sure, a typical overdrive could probably do the job, but one of my favourite often-overlooked pedal is a Treble Boost.
Perfect to match any British style amp, something like a Moth Treble Boost really pushes the front of the amp and you can even add a bit of Heat to it. With the Range control, you'll be able to select which frequencies you want to add to your tone and make your amp really scream.
Garbage drive may also be used, with a bit more controls than the Moth, you'll get a nice lot of volume, grit to push you amp, and full EQ control.
4 - A combo pedal? Why not...
If one option isn't enough for you, maybe we have something more compact in a 2-in-1 enclosure.
GrindR and Honker Hot Rod in the same box? We even have one with a Parallel toggle switch, letting you push each FX in parallel instead of serial as you would normally do. So have a look at the Mad Honker.
If you need a smaller unit, with less features but also a small footprint, the Baby Honker is the same as the Mad one, but without the parallel option.
More volume? The Mad Chiou is a Chiou + a GrindR in the same box. If you've missed our friend Kai Down video, it's your chance :
Conclusion
To conclude, there's a whole lot of stuff in this world to help you achieve the tones you're looking for... just stay open to everything and keep an eye out. Maybe the next best thing for you is on your friend pedalboard, or in our Shop, who knows!
Cheers :)
Guillaume
Some more reading :
