Disclaimer: I am a consultant for SPECTOR and I designed/manage their website.
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To be fair, I think QC can be broken down into a couple of categories:
Custom-shop-hand-made guitars
and
Mass-produced guitars.
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In the Custom-Shop/Hand-Made department I've yet to see any that didn't have amazing build quality. From Alembic and MTD to Fodera, Pedulla, SPECTOR, Ritter, Sukop, etc... these basses are either completely or mostly hand-made (CNC routing for pups and control, neck cavities for bolt-ons is very common even in custom-shop builds these days).
So you'd expect that if someone like Stuart Spector is signing his name and hand-numbering/dating the bass; it had better be near flawless. That's the ultimate in quality control.
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You have the mid-range Europe Shops which mix CNC work with a large amount of human-hand-ons construction. SPECTOR's Europe Series, all of NS-Design's instruments (which are made in the SPECTOR Factory) and the German Warwick stuff... it's all usually great stuff. There can be finish issues or as someone recently posted about Warwick, some strange inconsistencies with tone or wood quality... which is bound to happen. But again these guitars are majority hand-made and therefore I guess "custom-shop" to a level. And the quality will be really good.
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It's the mass produced import stuff that I think a term like Quality Control would best be applied. From China to Indonesia, Korea to Mexico... there are a slew of manufacturers that have introduced "entry-level" lines that are made in these countries.
Cheaper materials of lesser quality, cheap machining and labor costs and the ability to automate a large amount of the build process make these the most common instrument you're likely to come across when buying new or used guitars.
And the QC in many factories is spotty. It's the Monday/Friday curse... if it was made on Monday it's gonna be a lemon because everyone is hung-over from the weekend or doesn't want to be there (because it's Monday) or if it was made on Friday it'll be a lemon because everyone wants to finish quickly to start their weekend. Everybody's workin' for the weekend syndrome.
I've found that the Korean factories are building the best stuff coming out of Asia these days... and I've personally owned a couple of MIM instruments and they've been fine for the little amount of money they cost. I actually had a USA Custom-Shop electric guitar that wasn't $1800 worth of nicer than the $300 MIM model I had sitting next to it on a rack.
I think from manufacturer to manufacturer you'll find a difference in QC which is based on their production size (the smaller the easier to control), the country of origin (some countries are easier to work with than others), and the most important thing, customer perception.
In the end QC is meaningless if a guitar is put in the hands of someone who because of past experiences either loves (and is therefore willing to overlook some small faults) or is dubious (and therefore will be looking for any and every flaw) of overseas quality.
I'm a marketing guy... I was taught in school that if you can help formulate the customer's perception of the product BEFORE they get it, things will go much better so long as you're able to form a favorable impression.
Sad but true. Marketing is less about truth and more about hype and or psychological manipulation. And I retired from marketing years ago because I couldn't stomach the idea of helping people sell products I wouldn't buy for myself.
I came out of retirement to help SPECTOR for they are the only company I've ever worked with where the owners really care about the customers first and they strive to build/create products that are the best that can be had for the money. And there are a few bass manufacturers I can say that about in all honesty. Its a great industry with great people. And why, because bassists are cool.
