As I recall, the 3886 is good for at least 60 watts. However, there are two reasons the Niles amp may be rated lower. One, the power transformer may not be capable of the current necessary, especially driving a reactive load with a low impedance. I've heard of 4 ohm speakers that can dip to 1.5 ohms at some frequencies. Two, the heat sink capacity of the chassis may not allow for the required dissipation at higher power. Your goal requires 50 watts from each device, which the 3886 will do with the right power supply and heat sink. With +/- 30 volts, you'll need about 4 amps for each channel, a bit more those speakers with the low impedance dips. If the transformer will handle that, the next question will be if the heat sinks are big enough. My rule of thumb is the heat sinks should not get hotter than can be touched without burning oneself (they can get hotter than that, but that's a safe limit that will not overly shorten life expectancy). If the heat in your setup does push that, you'll need to resort to bigger heat sinks or fan-forced air flow.

True, 1% THD is a common yardstick especially with IC amps. Inflates the power out a little, but really the amp is starting to be over-driven. Your speakers will thank you if you never push the amp that hard into them. It's advisable not to use the amp in the part of the output curve that curves sharply to near-vertical.

Sounds like a fun project. I'm interested in how it turns out.

Bill