I've got a bit more information, I'm starting to feel sorry for the customer service person who helped me out here.
At today's conversion rates that's $146 just for the tubes and another £192 dollars to fit them.
The last part of the email sucks:
"I'm not surprised you want to proceed how you have suggested, there isn't actually anything I can do to make you pay anyway. I thought I would I contact you to let you know what had happened and see if you would help me out. My manager is going to come done hard on me for this mistake."
I won't leave the guy hanging, got to do something I guess.
In my own opinion I'm a bit annoyed how the company the head got sent off to takes 3 months to identify a tube issue and then charge enormous amounts to fix it. Then this guy is going to take it in the neck via his manager when all he's really done is the right thing and provided some great customer service.
I still think you are on a sticky wicket, but.....
Employees trying to garner sympathy with customers sounds a bit fishy to me. Sounds like someone is trying to turn you over a little bit mate, don't get drawn into feeling sorry for people that have let you down. I can't believe that you aren't already talking to the manager about this. Leaving an amp in the store for weeks before sending it for repair and not telling you the cost of the repair before it is undertaken is not good customer service. Lending you an amp is pretty cool.
Have you actually e-mailed Engl about this? I'd write to them whatever the outcome. In fact I would get hold of their customer service dept and visibly copy them in on every e-mail to the dealer, and e-mail the dealer with a run down of any discussions every time you have verbal communication.
Look, somewhere there has to be a warranty from someone that you can download whether it is the Engl website, the dealers website or whatever. Every retailer and manufacturer have to print their terms and conditions. If it wasn't explained to you and the warranty wsan't given to you at the time in a manual or something then I think that you have a reasonable case to say "show me where it says that the valves aren't covered".
Also, I'd be refusing to pay 50% of that breakdown. Parts at £92 sounds like a retail price and labour at £120 seems high to me. Carriage and VAT are obviously fine. Did they have to do more than just the valves or are they trying to make a profit over and above what the repair guy is charging? If it was popping from pretty much day one then getting progressively worse could it be that it wasn't biased correctly coming out of the factory? I would like to know exactly what they have done, and why the labour charge is so high. It can't take that long to change four valves and rebias, surely.
At this point, I'd be talking to the manager and saying the following things:
Where does it say that valves aren't covered by your warranty? In future you ought to be telling your customers up front. I believe this was misrepresented at the time of purchase.
If the issue is truly not covered by a warranty, then why exactly wasn't I contacted with a quotation for repairs before this work being undertaken?
I am unhappy with the quality of the product and your service as a dealer. I would like a full refund. I'll take my business elsewhere.