The PowerPlant to which Tiofelon refers will overheat. That's the main issue with it; and with the P500, as well.
This sounds like a bunch of rubbish Jburidan! The Power Plant 300 that OP is asking about has NO ' cooling circuit' that you refer to in your post. It is pasively cooled - the top plate is what The power transistors are bolted to & the sides have shallow fins for cooling. I'm not sure but I believe that the P600 is also passively cooled. If you owned one or more of these you know this already!
To the OP: I bought a used original P300 from the original owner & this is a 1999 unit that did. not have Multiwave. All the internals were original & the unit worked like a champ from the day I received it. The bulb highlighting the company logo was burned out, as expected. I bought this unit for TT platter speed control so I NEEDED Multiwave ASAP. Do I shipped this unit off to Rick Cullen of Cullen Circuits & he upgraded it for me for a decent cost. This unit continues to work like a champ today. I have all my frontend components plugged into it & like Elizabeth I'm careful to keep it loaded well below its max limit. IMHO it's a great unit. Time does NOT diminish its efficacy in providing regen power. Like Dan suggested, get it re-capped & you'll be off & running....