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?
gte357s
I've had the problem in the past and it was the output tubes. It's time to change the 6c33c's out.
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.
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 www.partsconnexion.com.
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.