My short response is "what zooombiex said" but here's a bit more.
By clipping, I think you mean overdriving the output section by running it hot but this is a good opportunity to contrast that with something else that many people call clipping (I'm not sure that's accurate but in any event ...) which is to slam the front end of your power section with an extra hot input signal. I.e., the old "power amp overdrive" versus "slammin' it in the red" (the olden term for when you pushed a console with a too-hot signal and the input signal gauges would move past the green part of the spectrum into the red "warning, you're overdriving the console's input section" portion).
Both can sound good or bad depending on the gear and the context. As just one quick example, the acoustic guitars on Obladi-Oblada were recorded by slamming the console into the red. Normally, with acoustics, that sounds awful. The Beatles, Martin, and their recording personnel made it sound wonderful - an effect unto itself, if you will.
With tube amps, more often than not, both types of distortion can sound awesome. Whether it's slamming your tube amp with a SS Tubescreamer pedal or cranking your blackface Fender amp to 10 (as Hendrix did on some of his recorded solos many people still think were recorded with Marshalls), it can sound great.
With the RV300, it's really easy - too much so, perhaps, to "slam it into the red" with a hot input. IMO (YMMV), the distortion you get by doing that to an RV300 is not especially pleasing. That's why it's so important, I believe, to drive yours with a quality preamp that includes a rear signal trim pot in addition to a front panel volume or gain control (it may be labelled either way). The good units, like SMSs and FYDs (I now own one of each), come with rear trim pots. On my SMS, for example, I run my rear trim pot around 1PM. That allows me to feed the RV300 an input signal it can handle cleanly.
The beauty of McI overdrive, I've read here and elsewhere, resides in the magic that occurs when they're run loud enough to produce output stage overdrive. With my RV300, to be honest, even when I've gigged outdoors before a couple of hundred people unmiced, I've never had to turn it up above slightly over half way up. I.e., it's damn loud! So I can't help you with a description of what the RV300 sounds like clipping in that fashion. I can tell you that, for most users, it won't be a practical way to generate overdrive and you'll be better off in my view using an effect pedal for that. Though now that I think of it, I've never personally tried an RV100 - maybe that would be a better option for this (worth making another point now - we often buy more amp than we need or can use ... then we wonder why, with the amp's volume at 3, it just doesn't sound that good ...).
Just my 2 cents. I'm not an engineer or tech though there are plenty who are who post here - perhaps some will also chime in.
"For me, I think the only danger is being too much in love with guitar playing. The music is the most important thing, and the guitar is only the instrument." Jerry Garcia