I've posted my experiences with some of the Hafler DH series upgrades myself. From an historical perspective, there seems to be 4 different categories of upgrading of these amps. Let's step through them:
1) 80's vintage POOGE: The Audio Amateur magazine published a series of articles as the DH series hit the streets for inexpensive ways to upgrade the amp.
There's lots of discussions in the blog-o-sphere about them.
2) DIY component swapouts: If you're comfortable with it, you can go in and replace the small-signal semiconductors and capacitors yourself. There are some good threads here at AK and over at DIYAudio on the topic. Get a 6-pack and spend some time searching to get an idea. Vendors like Qua-co and others have spotted this as a market and are producing parts kits and instruction to help remove some of the mystery of this approach. This has allowed someone with good de-soldering tools and skills to do basic remedial work on the good 'ol Haflers. I also see other brands like Adcom and other similar amps being supported this way. These usually are accompanied by replacement of the RCA jacks and the 5-way binding post speaker connectors.
Generally, these are component replacement exercises and there's no changes made to the original circuit design or layout - all you're doing is refreshing the 30+ year old components; some of which do not age well like electrolytic caps.
For our OP's question, $75 will get you to this level.
3) Power supply and rewiring changes: In addition to the refreshing work on the driver boards, you'll also see power supply upgrades that can include wholesale swap-outs of the all components including the power transformer. These can be daunting because they will require rewiring. Getting the grounding right can be a challenge unless there's clear direction from the parts vendor on how to install them. I see both Qua-co and Musical Concepts offering kits in this area.
The implication, here, is that we've gone beyond just remedial work and moved into more complex work that involves wiring layout and more complete circuit fabrication. In essence, you're now beginning to have a unique amp - not just the Hafler that you started with.
4) Wholesale replacement of the driver boards: Hafler made really effective and well-laid-out chassis and heat sink systems. Frankly, they're worth purchasing if the whole thing has smoked because the chassis are excellent DIY components. The last level is, then, to completely abandon the original Hafler amp design and go to a higher performance amplifier using a different circuit topology but using the original FET output devices and chassis. These usually come in the form of new driver boards and associated component/wiring changes to the Hafler chassis. I've reported my very positive experiences with the Musical Concepts driver boards over that last 10 years here at AK. I see other vendors have stepped into this level for other amps -
most notably the WOPL Phase Linears.
The good news is that these really do raise the level of performance for the amps. The bad news is that you'll likely _not_ get a good return on investment if you choose to sell it later. This means that you should plan on keeping the amp long term if you choose Level 4 upgrades.
Hope that helps put things into perspective.
Cheers,
Davd
