Bigger amps in an enclosed cabinet?

Each manufacturer should clearly explain in their literature if their equipment has any special ventilation needs. Many have their owners manuals available online or will answer questions if you email them.

As such, I'd suggest your get your info straight from the horse's mouth.

Heat is the only issue other than a strong enough cabinet to handle the weight.

I've got a really cool old Philco TV cabinet that lost the TV before I got it. It's my audio cabinet right now with only a 120W integrated and a CD player. Doors open it has no problems with heat. It's got plenty of room and I'd forseen adding separates to it one day. I'd also thought of the heat issue so:

Does anyone use the small box fans (muffin fans) to move air into or out of a cabinet? If so, can you recommend a really quiet one? Some of them are real howlers.

I swopped out the original [made in Taiwan] fans in my ARC 120 & 150 amps for Pabst Motoren,Germany, type 4840n fans.
These are small fans and very quiet. In fact I was pleased to see that ARC have started installing them in my new Reference 210 Mono amps., and this also means you should be able to find them in the USA.
Of course you will have to fab some brackets or grilles to mount them and consider air flow direction, etc.

Is your Plinius integrated Class A? If not, you will be amazed at the heat generated by a large Class A amp like the Pass or the big Plinius'.

You can use a simple computer fan run off a wall-wort type converter. I use two with trade name "Stealth" and they are quiet to the degree that I never even notice that they are on unless I look or unless I check the on/off switch. They run 24/7, since my amp is on most of the time. One tip for quietness is to run the fans at a voltage somewhat lower than rated (e.g., if the fan is a 12V fan, run it at 6-9 volts). I purchased a variable voltage converter from Radio Shack and wired an in-line on/off switch. Also design openings in your enclosure so cool air is sucked in from the lower level of the enclosure and the warm air is blown out. Alternatively, you can simply point the fans to blow on the heat sinks of your amp, but still allow an opening above the amp for warm air to escape. On my cabinet, the openings are on the back where they cannot be seen, and cool air comes in from the space between the double doors in front, blows across the amp then out at a little higher level in the back. The fans really made a difference in temperature within the enclosure.