Modification Nightmares

Rather than focus on modification nightmares, perhaps it might be helpful to break down your request into specific categories, and rate each service provider on a scale of 1-5, with 5 being the best?

Some categories might include:

Quality of workmanship
Quality of sound
Description of modification to be performed
Service
Communication

You cannot go wrong with Modwright. Disclaimer: They are a client, but I have been dealing with Dan long before he ever became a client. He is talented, honest and I have experienced him only as a person of tremendous integrity, both before and since I've started doing some work for him. I've owned many of his components over perhaps ten years, and none have failed to please me or justify the expense (I did purchase some used, and others new). On Grant's suggested scales I cannot think of any instance where I would not rate his work at the top. Given my connection you can take this however you like, but that's my .02 cents. As far as mods go, I have also had one very positive experience with Great Northern Sound and felt that mod was worth the money (ARC LS2B).

jax2 is correct. Mods done well are a very worthwhile improvement. If you can not do them yourself, then the aftermarket folks are a good way to go. the only word of caution I will offer, is you usually won't get your money back out if you mod a cheap piece and then try to sell it.
More expensive gear will usually give you a better return.
Also depends on what mods were done.

As an example, I have cheapie Parasound Zpre1. Sound quality is so-so in stock form. Add 400 dollars worth of vishay resistors to it, upgrade the coupling caps; and another 150 bucks for an oversized outboard power supply, and all of a sudden you have a reference quality line stage pre amp, that will compete with some really serious gear.

Now if I were to decide to sell the mod'ed zpre, I doubt I would get much of my money in parts back out of it, since everyone thinks its just a in house music piece.