Turk -
I'm sorry to hear that you experienced bad tech disease. The sad reality of the world is that diagnosis and repair (of anything really) is an art and that there are very few artists around. I have a lot of high-tech electronics around home and I really cringe when I know something isn't right. Replacement would be expensive; trying to get it fixed, probably too expensive and unlikely to really solve the problem. :-(
The good part about your rig is that it couldn't have been better designed or built when it came from the factory. The bad part is that it's probably an order of magnitude more complicated to work on than most solid state amps, and that fact that it's filled with vacuum tubes makes it really expensive to work with as well.
Under normal wear, it's possible that it could be all the tubes just getting old. For that (the tired amp syndrome), replacement of tubes (about 18 or so if I remember correctly for the Bass 400+) is the right place to start. But when you get a catastrophic failure it's more likely that another component, perhaps a cap or even a resistor has failed. This throws the amp circuit out of whack, gets you weird distortion and non-linear response, and ends up frying some of your tubes. A tech that's not experienced with working with this amp may only see the fried tubes and hope that replacing them will fix the problem. All that will happen is that it will repeat, as appears to be your case.
Even an amp tech that works on Marshalls and Fenders will be scratching his head with the Bass 400+. Guitars amps are a lot simpler - 2 or 4 output tubes in the power section instead of 12 that need to be carefully matched. To set bias and get the tubes working properly, they need a suitable speaker simulator or load (tube amps have no output with no load). If you use a Power Soak for Marshalls, your Boogie will reduce that to a pile of smoking carbon before it's even coming close to stress. I've got old SVTs as well. I love the sound and can tolerate the weight, but getting it worked on is a nightmare - lots of otherwise talented people say they can work on it, but results have been pretty spotty.
If a component in the power amp got cooked, the bias of the output circuit may be off. The tube associated with this part of the circuit may operate cold (which gives early distortion) or too hot (which gives a different kind of distortion and causes the tube to fail more quickly). The tube will test bad after being in this environment, but putting a new tube in will just kill it again. The tech needs to test for this problem individually on each tube.
You may also be aware that Boogie amps don't have adjustable bias. In a Marshall, when you have a partial component failure, the bias voltage may shift, but you may be able to adjust a trimpot to fix the problem, at least temporarily. No dice on the Boogie. They have a proper bias voltage that the tech can measure for, but to prevent your needing to have a highly competent amp tech, Boogie made the decision a long time ago to set a fixed bias voltage, tell everybody what it was, and require you to buy tubes that were pre-tested to operate properly at that voltage. If you have the wrong tubes, it won't work optimally just plugging new tubes in. Old tubes could tolerate a higher bias voltage than new ones, too. A good amp tech will confirm the performance of the circuit, use the right tubes, or can make the adjustment to the circuit to adjust the bias if necessary (generally, this requires substituting a big huge resistor for each power tube).
Since this last fix was clearly a boner, the tech needs to live up to his screw up. Asking him to fix it more when he doesn't know how to probably won't help (make sure that your work wasn't done by somebody else in the shop first). If they can't get it fixed properly, ask for your money back. If they won't do that (grrr, all those tubes), then see if you can at least get your labor back. Find somebody else and get it fixed right - it sounds like your in NYC, so you have a better chance than most to find somebody good. If that doesn't work, send it to Boogie. Unlike most amps out there, this one is probably worth it if it's not something really catastrophic inside.
One other piece of advice... If you have to drive to work everyday, it would probably work best if your only car wasn't a Porsche. Perhaps a bland but super-reliable solid state amp would be a good gear addition, if for no reason other than a backup amp. I pick Porsche intentionally for comparison. I've owned a couple including one now. They are really well engineered and really well built, as well as being sublimely sporty. When things are good, you really can drive them every day without worry. But when they break, you can expect so not see it for a while and it's going to cost a lot of money to get back. So, you just need to balance whether this is the right amp to depend on.
Best of luck, and never give up.
