Yet RedWine did the work to discover it and it does take a couple hours from in to out to do it right and test. They also direct connect from the amp to jack and not just bypass the resistor. Paid for labor isn't cheap and if there's a failure, who covers that elsewhere? You're likely out a DAP. It goes into the price. If I'm in the States, I would just give it to them if I want it done.
I'm not saying that RWA is bad. They get my applause for discovering this mod. But $250 just seems like bit much. If it was something like $150, I would happy do it. $250 seems to be pushing the limits a bit.