Since I just did one of these today I figured I'd post a "how to convert a SRM-T1 to use ECC99's". Not much to this really since the ECC99 is very similar to the 6CG7, the heaters draw a bit more current but the transformer seems to have no trouble with this. The only difference is the setup of the heaters since the 6CG7 is just a 6.3V unit but the ECC99 is a 6.3/12.6V. Easy enough to accomplish, cut two traces, add two wires and bridge two pins on the sockets.
On the top side all the anode resistors have to be changed for 30K/2W units. Rebias and you are done.