[AmpHead]

What kind of tone are you after? Adjusting levels between the guitar, any effects you may be running in the signal chain and the amp controls can minimize hiss.

What channel are you using? Do you have anything plugged into the FX loop? What are the controls set at?

Hiss is a product of resistive type components in the amp's circuitry and it's volume is dependent on the gain in the signal chain. There are likely some ways to set up the amp that will minimize the hiss and still get you the tone you want.

If you implement a lo pass filter with its 3 dB point at 11K you may lose enough upper harmonics to significantly alter the tone.

A post-output transformer filter should be wired in within the amp chassis - lots of dangerous voltages in there so experience and great care are required.

Anyway, per the above you may be able to get the hiss to a manageable level by altering controls in the signal path thereby boosting the signal to noise ratio.