[Amps] OMPower 2000+ Heater voltage ? is 8.6 to 8.7 volt heater enuf?

[Edward stallman]

I was reading with entrust about the FU728 tube failure and heater 
voltage . I understand the heater voltage should be 9 volts , Checking 
my heater voltage on my new OMPower 2000+ It's reading 8.7 volts most of 
the time , sometimes flickering to 8.8 volts .... I've seen it as low as 
8.5 volts on occasion's . At 1500 watts much less ..This has been a 
concern sense buying the amp close to two months ago . I've had one tube 
arc last week that blew a small fuse near the tube while using it on SSB 
. The amp was tuned properly and I was not tuning when the fuse blew .

Am I safe with the heater voltage at  these levels ?

When the fuse went , I was thinking of moving the tap from 240V  to 230V 
but it was already on the 230V tap ... Thought about moving it to 220 
but left it where it was at . My line voltage reads 240  to 245.

Thanks for reading , like to hear your thoughts . Ed N5DG