Hmmmm ... I love my PlatPro but I use it strictly for DB. I wouldn't use it for electric bass, and certainly not to front-end an OTB500.
Doesn't mean it won't do a job for you, just hadn't occurred to me to try!
The very limited EQ on the OTB has frequently been a topic on Basschat and there's no denying it. IMHO the whole point of the OTB is precisely that it [u][b]IS[/b][/u] a one-trick pony - you either like it or you don't.
There are loads of amps and pedals that will allow you to shape the contours and boost the Hi-Mids etc, and if that's what you like to do then you should buy another amp.
If you want a classic, old-school, blues-rock sound (P-bass through a f***-off valve rig) but at a fraction of the price and a tiny fraction of the weight, then the OTB ticks the box. It's plug & play, and you can stop wasting your time twiddling knobs.