High End, and Rack Mountable

Atma-sphere
Manley
Benchmark (especially for a hard drive system...it's half rack and needs the other blank half rack to work with 19")
Space-Tech Labs as an option
Counterpoint - vintage
Goldmund - vintage
Audio Research - vintage
Mark Levinson - vintage

Computers for your hard drive system: Sweetwater creation station and many DIY computers...just find a rack mount case.

Lots more I'm forgetting about

I use equipment racks for my video system, including for the audio amplifiers that support the video system. (Mine are from Mid-Atlantic, but are the same concept- full sized enclosed racks with back doors, shelves, etc.)
I'm not sure how well these things treat the components sonically, though. Mechanical isolation? The lighter stuff gets bolted in and needs no shelf to support it, but the heavy stuff is not just bolted in, but also sits on a pretty substantial shelf.
Having said that, I just bought a Mac 2700 as a gift for a friend with a pair of Sequels- it was pretty reasonably priced- and is fairly bulletproof. It should have the power to drive your Magneplanars, and, I'm told, is a sonic sweety.
The other thing is, once you get to using bigger tube amps in a rack, you've got to cool it with fans which contribute their own ambient noise. The Mac, being solid state, should run without cooling, but don't quote me, since I don't have one installed in the rack. (I do use a recent multichannel Mac for a couple of the surround channels, but don't run it in the video system alone- it is on with other tube amps that do generate some serious heat, and yes, my racks are fan cooled).

Brystons love Maggies. Personally, I'd buy 2 of the Bryston Pro Pak 500 watt mono plate amps and mount them to the back of the Maggies. Forget speaker cables, and just run long ICs.

As for the rack thing: I used to do that back in the day (mid 80s to mid 90s) both for audio and HT, but it can be a hassle, unless you take Barry's advice (but then the use of a NAB-type rack becomes moot) It is definitely hard on equipment cosmetics, and unless you use insulating washers on all mounting holes, tying all the chassis together electrically can cause major ground loop problems unless you are able to connect everything together using balanced (XLR) interconnects.

Since the harm from floor-borne vibrations are negligible in comparison to the far more harmful air-borne and internally generated vibrations and resonance it is actually a very good thing to direct couple the components to the racking system.

In other words, if you did not direct couple the components to the rack, where do you suppose the air-borne and internally-generated vibrations and resonance captured by the components in a moment in time but only disapate over a period of time would go?

However, it's really a moot point considering the design of the rack you describe since it is on casters (not appropriate for direct-coupling) and since the rack was probably designed for nothing more than perhaps an industrial look, computer server consolidation and convenient and controlled access.

Moreover, the rack walls and access door may also rattle at louder listening levels thereby making a bad situation worse.

I would suggest that you not attempt to rebuild your otherwise nice system around this rack that was never intended for this purpose and that will probably induce more sonic harm than what you had previously.

-IMO

Another consideration is using plastic or hard rubber isolation washers to not scratch the components when mounting.