Diagnosing a spitting/popping channel- need some help!

Did a search here and I think I am on right track, but might need some help here. Getting a sporadic spitting/popping sound sometimes in left front monitor speaker in a 5.1 setup.
I have the following signal chain:

• Pioneer HT receiver
• VAC Standard Preamp
• Odyssey Audio Monoblock Extreme amps
• Monitor Speakers are ACI Jaguars and there are dedicated ACI Subwoofers for each channel.

I use the Pioneer HT receiver L/R preamp outputs into the VAC preamp HT bypass inputs for when I use the TV so the Pioneer HT receiver amp sections are out of the chain for the L/R fronts.

Have done the following troubleshooting:

1. I switched the preamp out L/R cables and the spitting/popping continued out of the same left front speaker (rather than switch sides).
2. I turned off the Pioneer receiver completely and still get the popping.
3. I assume that means it is not the Odyssey amp; and not the Pioneer so it must be in the VAC preamp?

Am I right in assuming it is likely a bad preamp tube in the VAC?

The VAC manual says it has the following two pairs of tubes in the Line stage:
2 x 12AU7 & 2 x 12AX7 (line stage)

Where/how do I start checking tubes?

Am I on right track here?
Thanks.