This is an ad by Kavi Alexander, one of the most respected hi-end recording engineers in the business. He left a phone number. Why not call him and see what he has to say?
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I suspect you would be pleasantly surprised by the performance of the upgraded Quad 405 (and I oughta know - I'm the guy who started modifying them back in the late '70s). I also expect that you would get a lot of "bang for your buck" compared to the cost of buying another amplifier. On the other hand, there is always some risk when sending a component off for modification. The site www.quadmodsusa.com/405upgrades.html appears professionally done and offers the required services, but I know nothing of their reputation.
If you have not already done so, Google "Quad 405 modification" and you will get a lot of useful info.
Rebuilt Quad 405s have made a lot of audiophiles happy - it's a very good amp (and really a LOT better than the stock amp). Like a lot of other things in life, though, there is no way to know if it's going to be everything you want without trying it. Good luck!
We restore QUAD Amplifier as a hobby. In our experience, the QUAD 405 works best with high sensitivity speakers e.g. 90db/m or better. Their output is very tube-like and do not cause listening-fatigue as other semiconductor Amps do. A power supply and audio boards restoration does wonders to the sound.