so just a quick question
is it true that if you use a multieffect on a tube amp it will mess up your sound

cause im about to buy the boss me-50 and i have the marshall dsl-401 tube amp
and ive just heard some rumoers about it
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It won't necessarily mess up your sound. Multi-effects pedal like the POD sound much better through a tube amp than a solid state, hybrid or modeling amp. The absolute worst sound is multi-effects pedals through a modeling amp. The best sound you will get are good old analog pedals and a tube amp. These are my opinions based on my ear.
the digital pedals in general never sound as good as analog pedals. And with any kind of multi effects unit they are all digital. But the only thing thats really wrong with them is that they sound just like they are called, digital.

So ultimately it will make your sound much more digital, but it wont "mess" up your sound like it sounds like you are thinking. Multi effects pedals are really nice if you want to save money on a lot of pedals, and/or want to save space, like if you are in a college setting.

So if your looking for that perfect tube amp tone, it will irritate you a lot... but if your looking for a lot of effects in one unit, and are willing to sacrifice a little tone, then maybe you should go for it, its your call
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It depends on what you're using the pedal for. For vintage style fuzz effects, it's probably a no-go. But for delays, chorus, flange effects, etc. you can't really go wrong with the higher end boxes. I've used the GT-8 with several tube amps, and the tone is still there. The tone-sucker boogy man won't come to get you unless you don't set the unit up properly.
If you're using any sort of distortion or overdrive effects from your Multi-Fx, you want your amp to be something loud and clean like a Twin. Otherwise, the likelyhood of it sounding like ass will multiply by a great degree. But contrary to what many people on here would have you believe, digital effects don't send little boogy men that go out and "steal" the life and tone from your amp.
i really think that twostring has the right idea here. multi effects (and digital effects in general IMO) are better at modulation and time bessed effects than they are at distortions, fuzzes and overdrives. if you get a good to great multi, you shouldnt have much tone loss (especially if you know how to set it up). sure it may not sound as warm as a pure analog stomp box, but then again that isnt always what you want (some people want pristine repeats on their delays). but for a box to do gain, i think that you want to avoid digital. but for the other stuff, a good multi will be fine if not very nice.

with your setup i would probably run the wah first, then the ds-1 set as a boost (gain low, level up) into the amp. then put the multi in the effects loop and use it for chorus, phaser, tremolo, delay and other modulation effects. run that back into your amp and you should be good to go. thats how i would do it anyway.