With the recent increase in popularity of 6550's, you may find that a set of new 7027's are less expensive.

A word of caution:
The heater filament currents of these tubes may be a bit higher than the stock EL34's. Have your tech make sure that the filament voltage is a healthy 6.3 VAC. Lower than 6.1 can cause premature tube failure.

And [u]always[/u] get the bias set properly!