I like ,TIM,Time machine boost and Maxon 820.
820 is a little noisey but has a sweet smooth type of boost.
Tim can get a little of the smooth or can be just a clean boost.
TMB very clear and has huge amount of boost ,some amp settings will have a treble thing that will need to be dialed on alittle but very cool.
These are the only ones I have tried but they are all keepers.
Keep in mind that the Flexi is an ultra ultra sensitive amp, so some characteristics that you don't even notice playing through other amps will shine through with the Flexi. Wether or not that is a good thing is your decision.
Having said that: I've tried a lot of OD's with my Flexi and found that non-true bypass FX are a big no no; you'll hear the buffer when the pedal is not switched on.
Also the so-called line-drivers didn't do it for me; it's as if you switch from humbuckers to single coils when you engage such pedals. This is even when you use only that particular pedal, I've tried this because I thought I maybe had too much tonesucking going on with my normal setup which didn't turn out to be the case.
I ended up with an Okko pedal with an added footswitchable gain-stage. It's very tweakable so you can keep your tone "neutral" if desired and it's 100% true bypass, absolutely no colouring when switched off.
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