Good to know that you have good experiences with the Platinum Pro.
I always use my own amp.
I think it is better to use the Platinum then a DI from the sound man because of the impedance mismatches as someone said before.
I never use the Platinum in the effect loop though as someone suggested here. Is that better then just plug your bass directly in the Platinum and going straight to the amp? or is it not possible to use the Platinum as a DI and also plugging it in the effect loop?
The sound man's DI is one that works with his system (and, most likely, many, many systems.) The reason for choosing it over yours would be that he is familiar and comfortable with it. If yours (the Platinum Pro) works equally well with his system, there's no downside to using it instead. And the upside is that it offers you features that his DI doesn't. I don't believe that you'll have to worry about impedance mismatches using the house gear, though.
A/f/a plugging into the "input" jack vs. the "effects in" jack on the amp: The input jack will send you through the preamp and EQ sections of amp, effectively giving you two preamps; the Platinum Pro's and the one in the amp. By plugging into the effects send, you bypass the amp's preamp and EQ sections, using only the power amp section of the amp. Sometimes, on an amp that is slightly underpowered for the situation, you can gain (pun intended) a little exta volume by running into the input section, as opposed to the effects in jack. I've done this with my GK MB200, when I found I needed a little bit more ooomph than plugging into the effects in would afford.
No matter how you plug into the amp, the DI still works (either way.) Also, when connecting (the Platinum Pro) to a console that supplies phanton power, the Platinum Pro receives its power from the board: no need to use your AC adaptor or batteries.
Finally, a/f/a "always using your own amp," I've found that sometimes discretion is the better part of valor: The fewer things that need to be moved, repatched, etc., the fewer things that can go wrong.(Murphy's Law) On a festival situation, things can be rushed, crazy, the sound guys get fatigued, high, whatever. Sometimes the path of least resistance -- drop your Platinum Pro on top of the "house" backline bass rig, plug 'n' play. In that most of the house sound is gonna be coming from the mains, your amp is mainly serving as a stage monitor. If you can live w/ whatever sacrifice in tone, fidelity, stc. that you may incur by using the house rig, you're paid back by less stressed, less mixed up, grateful sound guys. Not only do they not have to jump through hoops for you, but they don't have to repatch (much) for the next act when you're done.