I have my guitar pickup volumes and my pedal settings all very much how I like them at home, at lower volume in my Marshall, and at our rehearsal space at high volume, in their Mesa Dual-Rec.

We're playing our first gig, ever, at an open mic next week, and I'm supposed to just plug my pedals into their Crate VFX5212 combo (it's a v50 tube amp, basically), and just let 'er rip. Any advice on how to show up, without much of a sound check, and make sure my clean and distorted are at similar volumes, my clean isn't too boomy/bassy, etc, since it's a foreign amp to me? Note, it WILL be mic'd through a PA, and there's a sound guy there, but we don't get to soundcheck or test anything, just walk up when it's our turn, plug in, and go.

I was going to play my Gibson Les Paul out, but I'm starting to rethink it and maybe bring my Ibanez Prestige or my Jackson Soloist instead, since they'll have more variety of tones with their 5-way pickup settings, in case something doesn't sound right while I'm at it.

Thoughts? Advice? Appreciate it!
well volume should be okay as long as you arent switching channels on the amp, if you are doing it all with pedals the volume should be the way you are used to. As to the sound, the sound guy should be able to keep it under control if you give him some idea as to what you want.
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The amp belongs to the house band / host band at the open mic, right? Very typical kind of setup.

Bands that are hired to run these thigns are generally made up of people with decent gear and they know how to use it. It's properly set up. It will already be balanced between the clean and dirty channels. As far as muddiness goes, ask the owner of the amp if he/she minds you mucking with the controls. They might not like that, but will respect you if you at least ask instead of assume. Then there's always the tone control on the guitar.

It's pretty typical, too, that they'll give you a quick rundown on what is in their pedal board, how their channel switching is set up, etc. It just takes a sec and makes everyone happier.

Usually the host band will play a set first. Watch and listen. What kind of guitar is being used? Are the channels well balanced between clean and dirty. If they are, you're good. If they're not, you can frame your question like "I noticed the clean sound was really washed out. Could we take the reverb down on it and turn it up a notch or two because I need a nice clean sound for the song we want to do?"

Even as a more established band, sometimes you'll run into a stage manager who wants to keep his/her job dead easy and will insist that you use whatever backline is there. Best just to get used to the idea that - every now and again - you're going to be stuck with someone else's gear that may or may not be to your liking, and you just learn to make the best of it.

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

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Thanks for this advice guys - I should actually give more detail... there is no house band, it's just donated gear from some music shops (sponsors) in the city, to this popular bar (across from Wrigley Field). They've only done this 2-3 times, and we couldn't attend one in advance, which is why the concern.

Also, I don't plan on using the amp's footswitch, if it even has one (I don't believe it does) - I plan on living with whatever clean tone it has, PRAYING it's not too boomy with my Les Paul (i.e. if they don't have the gain and bass up too high), like when I plug into the Mesa stack provided at the rehearsal space, and then using my ME-50 to switch on and off distortion (basically just a BOSS DS-1) with level and tone and drive all set to Noon, and then I just leave the Noise Gate, the Compression, and an ever so slight Reverb on it on at all times, so I'm just setting my volume initially.

Seem like a typical setup? Any advice? Thanks guys!
I winced when I read that you were playing through a Mesa and getting distortion from a multi-effects.