Amp and preamp on same outlet?

That punishment is not severe enough. He should be stripped naked and hung by his toes and stoned to death by the villagers with VPI magic bricks and his remains hauled off for the vultures to feed on!

Not to mention havin his 24K gold plated Audiophile card revoked!!!

it's ok as long as you want to listen to tunes.

IMO; it really depends upon which amp and preamp you are using. You never specifically mentioned that.

Many ARC products are made to be used with a 20 amp IEC power cord, is yours?

I would try plugging the amp into the wall outlet and then run a heavy duty extention cord to your preamp (using whatever connection adapters necessary) and listen for yourself.

As Yogiboy mentioned, if the breaker has not tripped then you should be fine however the system might sound better if the amp and preamp were on different circuits or dedicated circuits.

Using separate outlets, especially if they are on separate runs back to the breaker panel, will reduce the amount of amplifier-generated electrical noise that may couple back into the preamp. On the other hand, doing that may increase susceptibility to ground loop issues, including high frequency electrical noise as well as low frequency hum. If the connection between preamp and power amp is balanced, the likelihood of ground loop issues is considerably reduced.

All of this is highly dependent on the design of the particular components, and has little if any predictability. So ZD is right, "there's no way to tell unless you try it." But if, as you say, "it sounds great as is," that would seem likely to be the bottom line.

Regards,
-- Al