Many designers do not think boutique parts sound different or better. Many big names that I won’t mention here think all wire, fuses, capacitors, resistors etc... that meet spec will sound virtually the same. They are wrong. Just because they are talented designers does not mean they were or are opened minded enough to test all manner of boutique parts in their circuits and relentlessly fine tune and learn.
In some cases, I won’t say all, it’s an amplifier designer’s ego that gets in the way. It’s like, I know all about circuits so my designs are perfect and don’t need any tweaks. Another concern is that the cost of incorporating various tweaks or upgrades like premium fuses and power cords and capacitors almost certainly means he'd have to compete above his weight class as it were.