Amplifiers in question are Dynaco Mark 6's. A full restoration of these beasts is on the menu, and since they arrived with one amp having mismatched and a couple broken 8417's, and the 8417's in the other amp appear to have already lived 2 complete lives, i'm thinking this is the time to change them over to 6550's.

Originally rated at 120w on the quad of 8417's, can anybody tell me how much power output i'm going to lose switching to 6550's?

All thoughts and opinions appreciated.

Also, can anybody point me to a reasonable and reliable source for 6N030T delay/relay tubes?

From tube data, the voltage to drive an 8417's grid is less than that of the 6550, so you can do it, but you may have to adjust the driver tube circuit to get the 6550 to work in the circuit.

Here is an interesting page:

http://www.triodeel.com/8417.htm

BTW, the perception of sound is relative… and a smaller amp with more efficient speakers can easily be "louder" than a larger amp with less efficient output devices.
Also, the human ear is a non-linear measuring device. As you might know, you have to increase the output power 10 times to double the perception of sound level. In general you won't be able to get the same output as the 8417s but a few watts difference may not be that noticeable.