Modified equipment...

Add the price of the mod to the retail price of the unit to come up with a combined price. Depending on the specific unit and who di the work it can definately add value. At the very least you should disclose the work done so that the buyer can expect a difference in performance (hopefully a great improvement!) and no that the warranty has been voided.

Leave it stock. While they're may be a plethora of great mods out there, if you intend to resell later, trying to explain the mods to someone unfamiliar with them is imho a pretty big turn off. Individual components are voiced to work as an system, or they ought to be, that's why you're dealing with a professional designer in the first place, right?
In the same vein, manufacturers who tout their great sounding components and then simultaneously offer an upgrade package including the parts that REALLY make it sound great are kinda goofy too. Yeah I know they're trying to hit their price points, but what they really accomplish is making their "unimproved" models instantly obsolete.
As a final note, it used to be a comfort to see any manufacturer who offered just one model preamp, power amp, etc. Ostensibly this was the best he wanted to offer the public. Integrity of purpose and integrity of design, if these things are important to you, don't fix it if its not broken.

I've modified my Rega RB300 tonearm with Incognito kit spending additional $400. Original arm price is $425.
Result:
1. I can only sell this arm for $400(+-) loosing more than half of my investment(basically mods only giving arm free)
2. It's still Rega arm that doesn't give you desirable high end adjustment options. It will be still Rega even if you will thread through SME tonearm wires or any other high performance wires.
3. The imaging had been greatly improved compared to the original arm but value had been decreased.

So what's the alternative?
Sell original Rega arm for $200~225 and get used SME309 for ~$800(there are bunch of excellent arms in that range becides SME) which is price of modified Rega with all adjustment options, greater performance and certainly better value.

In any other cases such as modyfying already engineered components through the company that did not manufacture them wasn't realy a value increase or a good investment.

Am I dumb after all?
-- Yes I am, but that's the audiogon forums are here for to let us fellows tell how dumb we are!

The moral is B4 mods think about aquiring a better component to compare the final values. ...sort of playing chess where you have to think 5-10 moves ahead.

Cheers, Mara