No need to get hostile....
Which 2nd stage are you referring to with this: "it's still less than half of the price of a new plastic 2nd stage that's based on the 109 (balanced/adjustable) design to begin with."?
Sure, I could have them leave the 109 as is and just get it serviced, but I really have no idea if the rubber parts are any good, it's been sitting in my cold, damp basement for the better part of 30 years. I also have NO idea what kind of performance to expect from it. I'm open to the idea of it, based on my earlier post, but I really don't know if it's cost-effective. ... if anybody's interested, I can post pics.
First, sorry if I came off as hostile, that's not my intention. The G250V and S600 (especially the G250V) are direct descendants of the balanced/adjustable, which is the 109 with a balanced poppet. IOW, if you upgrade the 109 to balanced/adjustable you'll have basically a G250 in a metal case. (The S600 has some added complexity in the breathing adjustment mechanism and a smaller case.) The metal case is FAR better in cold water and has some intangible advantages, like reduced dry mouth and MUCH better durability and toughness. Its not a coincidence that SP's current 'latest and greatest' is an all-metal 2nd stage. You already have the classic.
I'm not sure how to put this delicately, but the shops you're visiting are A) full of it, and B) just trying to sell you a new reg....that is, after all, what they do. I'm sorry that you can't seem to find a service tech that understands the value of the older regs. This is a really common problem; it seems that excellent, reasonable repair technicians and dive shops are the exception, not the rule. Maybe that's why the whole business model seems to be crumbling. Here is some accurate information about your 109: Once you upgrade to balanced, the repair kit is the same as the G250V and S600. The upgrade consists of a poppet, spring, and balance chamber. The retail price on those parts in the U.S. is less than $20, and you only replace them once. The rubber parts consist of a diaphragm cover and exhaust tee. Those parts in the U.S. are less than $30 retail. If you also need a diaphragm, a good guess would be $15 retail; but I have rarely seen 109s needing new diaphragms. Soooo....worst case, all parts at retail, $65 plus the cost of the repair kit. I've rebuilt dozens of them and have never spent anywhere near that much on one. Granted, these are U.S. costs, but there's simply no way the Canadian cost is double or triple. As far as labor goes, the 109 is simpler to rebuild, and if your tech is hesitant to do so, find a new tech.
The 108 is not as nice a regulator, but works very well for an alternate 2nd stage. It's the precursor to the current R190 series; unbalanced downstream design. It takes the same kit as the R190, R380, etc.... and is essentially one o-ring, a puck seat, and a nyloc nut. I only have one 108, but it performs better than any of the R190s or similar that I've seen, and mine is the less-desirable single adjustment. It took me a long time to get around to rebuilding it because I didn't think it would work very well, but I was wrong.
Do you have a drysuit? If not, you don't need a sealed MK5 or MK10. That's likely to be a controversial statement, but honestly I don't see how anyone could dive in water cold enough to freeze a MK5/balanced adjustable in a wetsuit. One thing the dive shop doesn't mention (they probably just don't know) is that the older BP first and especially the all-metal 2nd stages are more freeze resistant to begin with; there is better heat transfer and lower flow rates. But, sealing them is easy; just fill the ambient chamber with silicone grease (or PTFE grease if you are diving with higher O2 mixes). If a tech says he can't do that or even that it's difficult, expensive, or not worth it, guess what.....find a new tech.
Here's the most telling statement in your post: "it's been sitting in my basement for the better part of 30 years...the set up looks basically new." Doesn't that tell you something about the durability and quality of these older regs? As far as evaluating the rubber parts, just look at the exhaust tee and the rubber purge cover. Are they dried out and cracked? If so, replace, if not, they're fine.
There's another issue you should be aware of; SP in their infinite wisdom has recently abandoned the "free parts" for life program, meaning when you buy a new regulator, you will now pay absurd prices for rebuild kits just like everyone else; even if you stick to the excessive annual service policy.
Nobody can tell you what you like. If what you want is a new, expensive, plastic regulator, fine. But don't fool yourself into thinking it will improve your dive experience. The truth is that scuba regulators have not really improved in decades; only gotten more complex, cheaper to build, and more loaded with "features" and design modifications that make them appear better on breathing machines. This, unfortunately, tells you very little about what it's like diving with them, or living with them and 'abusing' them year after year on dive boats.
Okay, rant over......
