A tiny trimpot on the board for each channel lets you adjust the feedback loop value. This allows you to "optimize" the amp for your speakers and measureable distortion was said to be reduced up to 70db. Whether this adjustment produced any audible benefit was open to debate at the time. To make the adustment a small box called the XL-10 was given by Hafler to its dealers. About the size of a paperback book it had two potentiometer knobs, a single RCA input, and two speaker jacks. When attached to the XL280 music would play like a small whisper out of one speaker while the other speaker played at normal volume. You then pressed your ear to the quieter tweeter and using the screwdriver through a hole in the top cover of the amp made an adjustment. I laid the louder speaker face down on a carpet to make it easier toi hear.

While slowly rotating the trimpot treble sounds would go from a subtle tss, tss sound to a quieter shh, shh and back to a tss, tss. By fiddling you could find one position that sounded slightly quieter: that was the null point. You then changed the connections and did the same test for the other channel. The downside of this adjustment was that every different speaker pair required a new nulling. If you were friends with the local Hafler dealer you could drive down and borrow their XL-10. But few customers knew about this device and I suspect in the mid 1990s when Hafler/Rockford stopped selling to the consumer market that dealers simply threw the XL-10 in the trash or gave them away. The dealer I worked for in the 1990s wouldn't give me theirs when the XL280 was discontinued just in case an old customer wanted to borrow it. Whatever happened to it I'll never know.

However, I had opened up my store's XL-10 and sketched the interior. It contained a coarse and fine trimpot and a couple of resistors, simple. So I bought a small test box from Radio Shack, some speaker jacks, the parts, and built my own. It worked perfectly and I used it for years for my XL280. I wanted to include a photo here but both it and my schematic have gone missing. Keep in mind that the tool only worked with the Hafler XL280/XL600 amps. If you're still using its original boards no need to stress about yours, simply put the trimpot setting in the center on both channels and enjoy.

BASIC MODS
The XL280 is a great platform to use for mods because of it's six MOSFETS per channel compared to four on the DH-200/220. With 145 continuous watts per channel and 'dual mono' power supply its a compact and robust amp. One downside is that the original 35+ year old boards lack a DC offset adjustment meaning it can't be adjusted if the transistors are out of spec. As you can tell from my other Hafler pages I'm a big fan of Musical Concepts mods. So if in doubt I suggest upgrading the original boards to modern versions.

Below is a photo from about 2005 with four Musical Concepts 12,000uf 80v caps replacing the 8900uf stock versions. I had upgraded the stock board electrolytics and film caps.