I'm mostly into seventies rock, and am preparing to play with my brother the power drummer soon. This means I need to upgrade from my bs practice amp. What do y'all think? Something with tubes? What's my best, affordable choice here?
Depends where you are.

If buying used, it is hard to beat Peavey for the most part. Look into the Valveking and the Classic 30 or Classic 50.

I'd also suggest looking into Traynor YCV50B - a tone monster.

Marshall DSL40C new seems to be quite popular as well.
^+1 on the Peavey stuff. Also the Peavey Windsor might be a good way to go. They are out of production, but they are decent amps that go for a pretty low price in the used market. Orange makes some really good tube amps for classic rock, but they are pricey, even used.
Gibson Les Paul 60s Tribute
Jackson King V
Peavey Valveking 100
Ampeg VH140C
Boss TU-3 Chromatic Tuner
MXR ZW-44 Overdrive
Dunlop ZW-45 Wah
Boss NS-2 Noise Suppressor
Digitech JamMan Solo XT
Peavey Vypyr VIP 1
A used Carvin Belair 2x12 50w combo has a good classic sound, should be plenty loud, and won't set you back a hell of a lot. Here's one on Reverb.com:

A Fender HRD combo would be a good option, but it won't have a good drive channel. The new Fender Bassbreaker like corrects that.

Vox AC 30 would be good if you can afford it.

A beefy used Mesa or Orange can be had under $1000.

The relaunched Supros are nice, but may be too pricey.
Sturgeon's 2nd Law, a.k.a. Sturgeon's Revelation: “Ninety percent of everything is crap.”

Why, yes, I am a lawyer- thanks for asking!

Log off and play yer guitar!

Strap on, tune up, rock out!

Features (channels, loop, reverb)?

What tone (70s can mean a lot of different things)?

Combo vs head/cab?
70s? Grateful Dead? Bob Seger? AC/DC? Al Di Meola? Lots of very different guitar tones.
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY
8Len8 Here's the thing, to be honest. I played 30 years ago, then came a wife, kids, s***, you get the idea. I'm just getting back into it all. Just trying to pick up the pieces. Yes, it was a cop-out to say just "seventies". But that's where my heart lies. UFO, Zep, ACDC, Power Pop. That sort of vibe.
Didn't see this thread until today.... So, you may have already settled in on an amp. For the variation in the types, styles and genres of music you mentioned, also consider the Fender Mustang III and IV v.2 model amps.

The Mustang III and Mustang IV can really soumd good once you dial them in. Mlst of the factory Presets are very effects intensive. These amps can get very loud, but also sound very good at bedroom levels. They have 100 and 150 watts, respectively, and good 12" Celestion Speakers. They have an easy to use on-board LCD control screen, 100 Presets, 17 Amp Profiles, and 44 Effects, plus 1/4" TRS Effects Loop Send/Return jacks, and XLR Line Outs. There are plenty of amp shoot outs on YouTube to get an idea how good they can sound. They're also very reasonably priced, especially when marked down on sale for the 4th of July Holiday sales.

If you have a PC or Mac you can download the Free Fender FUSE software. FUSE lets you edit and control the amp, download/upload Presets from/to the Fender FUSE on-line community and more. If you have an Android Tablet or Smartphone the REMUDA App is the way to go. REMUDA brings even more features for full editing and deep amp control and has features FUSE lacks. Both software alternatives work with all the Mustang amps.
Mustang v2 III/IV●EXP-1●FUSE●REMUDA●Yamaha THR10C
Epi LP Florentine Pro●LP Cstm Pros●LP PlusTop Pro●Sheraton-II Pro
Cstm Strat Vntg Noiseless●Guild D-55
So Creek Cables●BOSS RC-1●RS7500
D'Addario Strings●Vari-Grip●Planet Lock Straps
TS, I dont think you would go wrong with a Tiny Terror - Combo, or Head and 1x12 Cab. I've had one for 6 years now. It's been gigged with, and used as a practice space amp. I made the jump to a vintage Marshall when I started playing bigger venues, but I still hook up the TT on 7 watts when I'm jamming in the apartment.

For some smaller ideas that you could look at used I'd recommend a Fender Blues Jr., the Peavey Windsor or Classic 30 (<-might be a bit loud). But if you're just staying on a practice amp there's something wholly awesome about an Epiphone Valve Jr and a couple pedals. It's classic, and you get to swap out the board a lot to have some fun in customization. If I didn't have a TT, I'd own a Valve Jr. I've also played the Egnator and Blackstar small amps - real solid little monsters - they will do you just fine.

I hope this helps!
1981 Gibson Les Paul Custom (Black Beauty)
1980 Marshall JMP 2204
Last edited by Jonny Ryan Mac at Jul 1, 2016,