Gassy tubes (tubes that glow blue) are not uncommon. That, by itself is not reason to change them out.
However, the sounding dull can be a symptom of aging tubes (among other things). If you are using the amp a lot, 2 1/2 years is not an unreasonable amount of time to need tubes. If you had access to a tube checker, you can test them one at a time to find the bad one (or ones). Without one, you could get a preamp tube (12AX7) and move it from spot to spot and see what difference it makes. The power amp tubes (6l6) really should be replaced as a set.
Or you could just replace them all and figure the old ones could be spares in a pinch.
On most tube amps, after replacing the output tubes (that's the 6l6s) someone should go in and adjust the bias. Bias is the amount the tubes are turned on with no signal coming in. The bias in a 6505 is set by the factory so cold that you really don't need to worry about it.
Note: On any tube amp, the voltages inside are rather high, probably around 500 volts DC in your amp. You
really don't want to come in contact with it. Use caution and standard precautions when you change tubes.
Always unplug the amp before working on it.
Make sure it has been off for a while. The filter caps can retain a potentially lethal charge for a half an hour (sometimes longer, depending on the design).
A lot of people work with one hand in their pocket if they are near exposed circuits (that keeps the voltage from going through your body if you get your hand somewhere it really shouldn't be).
It's really not hard but some caution is warranted.
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