Atmasphere is spot-on with his recommendations - these are wonderful little amps; well worth rebuilding, and best left in stock form. I've rebuilt a pile of them over more than a couple of decades, including some that were brought to me with various mod kits . . . and virtually every well-known mod actually decreased the stability margins to which the amp was originally designed. David Hafler certainly knew what he was doing.
Of course, one has to bring them back to original operating condition, and replacement of electrolytics and coupling caps are obviously mandatory (use the original values for the latter!). Don't forget that these were frequently assembled on somebody's kitchen table, so inspect very carefully for poor solder connections, and very carefully re-form any stretched tube-socket pins.
Back in the '80s you didn't have to pay too close attention to the resistors, but nowadays ALL vintage carbon composition types should be carefully measured for drift before assuming they're okay (watch for voltage ratings if you sub modern metal films). Don't forget the "Biaset" resistor on the output tubes' cathodes, if this drifts, your bias settings will read incorrectly. Replace those silly 10-ohm input grounding resistors with wire links.
The power supply is indeed the weakest link, and some simple revisions can improve performance and reliability. Increasing the capacitance on the plate B+ supply helps dramatically, but don't add any before the choke, as it will then develop an appetite for rectifier tubes. There's some logic to adding a second 5AR4, but to avoid more filament load on the transformer you'll then need to use a separate 5v transformer for the rectifiers, and put this and all the electrolytics inside the chassis . . . which then gets a bit crowded . . . to me, this can easily be the first step down the road leading away from the elegant simplicity of the original.
I personally have had great results (in both sound and reliability) with a solid-state rectifier in ST-70s. Standard fare is a pair of 1N4007 diodes, and it's easy to wire them onto an octal plug so you can leave all the wiring original, and pop a 5AR4 back in if you like. A well-chosen snubber network in parallel with each diode eliminates their noise contribution (10nF in series with 47 ohms is a good one-size-fits-all snubber solution). You can use the same to replace the selenium bias rectifier if you like; simply alter the values of the bias supply filter resistors to get the bias pots back within a comfortable range. The solid-state diodes do increase the B+ a bit, but that's no problem as new replacement snap-lock electrolytics are available with slightly higher voltage ratings than the original.
A couple of final touches are an inrush-current limiting thermistor in series with the AC cord, and replacement connectors. For the input RCAs, it's just a hardware game, or you can make some RCA-to-octal-plug cables and use those front panel "preamp" sockets (probably for the first time in the amp's life!). For the speakers, I used to use replacements for a McIntosh 8-channel amp (MC7108 I think) as a perfect drop-in upgrade replacement; maybe Mac still has these available.
In any case, the ST-70 is just as fun today as it ever was, so enjoy it!
