Looking at the photo, I would say three of them at least are on their way.

It tends to happen with caps that have a top insert when they are placed in a hot environment. Audion amps often look like that, although it's been shown to be superficial. I can't speak for those Quads but the fact they are squidgy at the top suggests it's just the plastic insert at the top which has become mis-sharpen. Once the new caps are fitted it would be easy to check the old ones out and see for definite

I hadn't taken full notice of the product portfolio Stan-the-Man has designed for. Some wonderfully endearing products he's done under the radar

My DMM only goes to a couple of uF so I can't check their spec. Given the caps in 306s suffer from heat I'd rather bin the 85C ones for 105C new ones and get greater capacitance to boot. Mis-shapen sacrificial plastic covers can't be a good thing, it suggests to me heat is an issue.

I've said many times that I've see two 405's for myself (plus others I've not owned) where the late 70's caps were fried at eleven years old, and not heat-thrashed either as loud hard rock wasn't then on their menu from new.

No offence to Quad, but the 405 and 306 designs was kept as small in footprint as possible and not just the heat factor, the capacitors then available weren't really small enough. It's different now I think and you could happily up the temperature to 105 degrees and double the cap value without compromise I think.

I've played around with supply cap values here to a limited extent, going from 7,000uF on the Crown D-60's to 22,000. The bass reproduction may possibly be ever-so-slightly more 'organic' in nature, but really, the main difference is that the amps take far longer to shut down once disconnected from the mains supply. The smaller amps I make seem to get by quite happily on 4,700uF caps and I've discovered it's the transformer capacity that really matters to speaker-driving ability in a subjective sense - I'm quite sure that the transformer and reservoir supply should be optimised as a pair really, but I don't have the maths either way to confirm prior to confirmation by listening.

In a full-wave rectifier power supply, the transformer VA versus capacitor value determines how quickly the reservoir capacitors charge, while the capacitor value and load current determine how slowly they discharge.  It's a fine balance getting it right but generally over-value transformers do no harm.

I can't tell what the VA rating is for the PTX; it's an ILP but they've stopped producing toriods. The PTX looks a decent size. Dave's point about case size is salient. Making the case the same size as the preamp and tuner was aesthetically a good move and no doubt margins were improved but I can't help thinking some vents would have been a good idea.

If you are worried about heat build-up try planting the 306 onto some tallish feet which should help improve air circulation under the unit a little. On a hot day (sitting on its original feet) mine could get quite toasty, however since placing it on taller, turned brass spikes I've not felt the temperature rise above luke warm.

I too worry about heat and elect caps. I once had a 2A3 heater regulator which the designer said was fine but I was never convinced. Due to the ripple current running through the caps they were constantly so hot I couldn't touch them. Ok they were 105C rated but it's asking for trouble.