If I wanted to buy one I would buy one with heatsinks. However up to 10-20W continuous the plate affixed to the chassis is fine. I consider it a perfectly linear power amp up to those power levels and with a significant headroom for transients and short bursts. If you need 400W continuous from a 400W amp, you are either listening to crappy noise or your amp is seriously underpowered because you have no headroom, hence no dynamics, and when the music has a transient the amp turns off or something bad happens. That’s why there are 3KW amps.
So. the 1ET400A bolted to the chassis is absolutely fine.
Hmm my excessive expectations for over-engineering....
I know that in continuous terms, I seldom use more than 1 W - perhaps 5 W on the very most demanding material.... and peaks would max at 128W or thereabouts.... (at 8 ohm equivalent) - but those peaks do imply a 2 ohm max of 512W)
My current ClassD amps are rated at [email protected]
Crown XLS2500... the Fans have never spun up... and the internal "heatsinks" - are negligible
So in terms of "reason" and "logic" - the heatsinks are unnecessary over engineering.
Here is the counter argument:
Class D amps are complex beasts, lost of tiny surface mounted components - difficult and expensive to repair at the component level - most repairs are achieved by replacing entire boards.
My 40 year old Quad 606 can be economically refurbished when it needs it (every 20 to 30 years?)
The Class D amps.... well their lifetime is likely to be between 10 and 15 years - after which they are probably junk, as their boards will no longer be available as spares/replacement parts...
If they are over-engineered in terms of heat-sinking - and kept at substantially lower average temperature for the components - their lifetime is likely to be substantially increased - potentially doubled or more.
Silicon component lifetimes tend towards a logarythmic curve when related to temperatures... yes they can run perfectly well without additional heatsinks... but drop the average temperature by 5C and you might find that the lifetime is doubled.
Just my 2c / thoughts