Almarro has poping sound, please help

Recently, my Almarro 318B develop a loud "poop" sound. It is not very frequent. It is intermittent, usually have it after 15 mins or so after it is started. What should I do? I bought the unit used, and I have no history of the unit.
Should I take it to service, or should I try replacing the tube? Is there a way to find out if a tube gone bad? If I am to try replacing the tube, which one should I try first?
Is the sound coming out of both channels or just one side?
If it is just one side, try swapping the left and right output tubes and see if it goes to the other channel. If so, it is a bad output tube. If you don't know how many hours are on the output tubes, the smart bet is to replace them to be safe.
I had a speaker that had a pooping sound one day. Very concerned, I removed its grill to make sure everything was OK. Sure enough, it had had, well, "an accident" with its woofer shall we say.
I've had the problem in the past and it was the output tubes. It's time to change the 6c33c's out.
Thanks. I think I will try to change the 6C33C then. I did a preliminary search on eBay, it seems it is all made in Russian? Is there any other better choice out there?
I've had an Almarro 318B for a few years now, and I've experienced popping on a number of occasions. Each time it ended up being a dying 6c33c tube...Ebay has decent prices, particularly when you buy a few at a time, but shipping to the US is pricey and I've been sent an entire order of DOA tubes (and didn't get a refund b/c I didn't test them quickly enough). Since then, I've had great results ordering from
I would highly recommend NOT buying your 6c33c's from ebay.
I have also gotten many dead tubes from there.
It's a cheap tube, so buy them tested.
Consider not only the possiblity of a bad tube, or a bad tube pin holder, but a bad solder joint. If you are careful (very careful, high voltage) you can take the bottom off and have the amp laying on its side and running and connected to your speakers, and carefully wiggle (without touching anything else) the wires, particularly those going to the tube pin holders to try and find the bad solder joint.

Or for a shotgun approach, with amp off and disconnected, you can take a soldering iron to all the tube pin connections or others and see if that cures the problem.