Kdl, I think the point is to put your 5 KVA isolating transformer in the service area where you won't hear a hum. For example, mine will go where the second water heater used to be and buck twin-phase 220V down to 120 single. (I may need another tranny to do the buck, if I can't find a 220-in, 120-out isotrans.)The feed from this will come, via shielded twisted-pair, up to my listening room. At least, this is now my dream :o)
Yes, 5 KVA is overkill, but for good reasons, as Sean explained above. Your system may be comfortable with 3 KVA or less, but a bigger transformer has a larger iron core and is less likely to saturate. Saturation is a big no-no, as Sean said too.
If you're in a place where you can't make this kind of mod to your electrical system, my take on what to do would obviously be to find smaller, silent isolating and conditioning devices and use them on individual components, starting with digital. Others may have better ideas.
I am enjoying planning this setup and looking for the gear. Cooking with AC seems a lot simpler than shopping for hi-fi. Sometimes a lot cheaper too ! I saw a tested 5 KVA step-down tranny on eBay starting at $185. It was only that much because it was in a lot with two others.