Hafler DH-220 performance and modifications

I'm not familiar with either of those mods, but have owned a DH220 and DH500.  While both are powerful and stable amps, they're not among the most transparent or musical amps I've owned, so I'd definitely consider pursuing some possible upgrades while you're inside of it. IIRC, years ago I think Musical Concepts and VanAlstine had some well regarded mods for those amps.  

Good luck, and please post an update regardless of which direction you choose. 

I have been using the dl-200 for a year. I would not touch it with any kit, it's just right. (the design of the back is horrible though, but that's not about the sound) 

I don't know if the 220 is profoundly different,

I own both and I would not do anything to them, if they have been serviced and working/sounding good. Don't go down the rabbit hole trying to make it "perfect". Accept it as it is. I like Hafler stuff. If you want a different sound, buy something else.

John Hillig of Musical Conceptsdid several levels of DH-220 mods, all were good. They seem to still be in business just west of the city of St. Louis. Some of his mods are more than just parts swap outs.

As a side note, if I recall correctly, a DH-200 and a DH-220 are very similar in design. The -220 just runs at a slightly higher voltage. But I may be wrong. I add this because I think modifications to a DH-200 will likely also work in a -220.

I built both a DH-200 and a DH-220 from kits back in the day and was satisfied with both.  As bpoletti said they are very similar and in fact, are nearly identical.  I thought the 220 sounded just a smidge more full but never had the urge to modify either of them.  Both are extremely robust if assembled properly.  That's why there are so many of them still around even if built from kits. 

That said, I feel grislybutter and kyguy are reasonable in their guidance.  If the amp you have works well, installing new capacitors and maybe heavier gauge wire is about as far as you might care to go.  

You don't say what you have for speakers, but if you have something on the more efficient side, Hafler could be a good match.  The 220 was only adequate with my Rogers Studio 1 monitors at 86 dB.  I replaced it with a Belles Model 1 that was a substantial improvement.

For financial perspective, there's a Belles 150 on eBay for $325.  Good luck and happy listening on your journey!