My noise/headroom/heat statements were based on measurements, not a personal opinion. Any time you take a line level input, divide it down to (I believe it's 1/30th) it's original level and then immediately gain it back up to unity, you've added noise. That's not an advisable approach in a console design, but the designers clearly wanted to share the gain stage with the mic preamp. Also, the current source in the 409 preamp board is notoriously hot-- it's a class-A preamp, after all. It's a common problem in the 460. Pull a 409 sub-board and the odds are, you'll find tons of carbon buildup on the pcb, and in many cases you'll find actual heat damage to the pcb.
That said, it's doubtful that in your rack-up you'll have the same thermal issues one will have in a fully loaded, 32 channel console with fully enclosed modules.
(edit) I should also add that designers of Live and Broadcast consoles frequently make the noise for distortion/headroom tradeoff. Headroom is critical in broadcast applications, and so it's really no surprise that the designers configured the preamp in this way. By noisy I don't mean it's so noisy that it's unusable, but the noise floor is high enough (compared to a modern recording console) that thermal issues can put it firmly into the noticeable range.