Huge gains can be had upgrading the resistors and caps with the exact same values. As long as you use the exact same values matched within 1% for both speakers you are good to go. Mills MRA are only $5-$7 each and are great. The Path Audio Line is better, but they cost $22-$29 each. Put those on the highs and mids and Mills on the bass if you have the budget.
Erik is right about Clarity caps as they sound nice. The Clarity CSA line is affordable also. You can do better with Jantzen Alumen Z caps. They sound wonderful and are not crazy expensive.
As far as resale goes, if done right and professionally, you will not have a problem selling them down the line. I have upgraded every single set of speakers I have ever owned and sold them all no problem. I always share what I have done and use it as a selling point. Take pictures of the work, before and after, and use them in your listing. I have sold 10 or more set of speakers. No big losses etc....It only takes that one person who understands the sonic gains that can be had with crossover upgrades done right. Many Aphiles both understand and have first hand experience with these types of upgrades so finding that one buyer is not an issue.
I understand why some think resale is a problem, but many of these folks have not actually upgraded parts and then tried to sell ... no actual experience.... just a reasonable anxiety about it.
When done right this approach to upgrading is one of the most cost effective ways to improve the sound of your system. If you enjoy DIY, then it is double fun and enjoyment. If you have not done this before, then find someone to help you or get it done by someone who won’t charge you an arm and a leg.
