Regarding the SIT3, an important point to note is that it has an extremely low gain (for a power amp) of only 11.5 db. Power amp gains are typically in the mid to high 20s.
Consequently it can be calculated that the SIT3 will require a single-ended input (it does not provide a balanced input) of about 3.2 volts to be driven to its maximum power capability of 18 watts into 8 ohms. If a digital source having a volume control is used without an active preamp it will almost certainly not be able to provide that much voltage, even with the volume control at max. And in many and perhaps most cases vinyl or other analog sources won't be able to do so even when used in conjunction with an active preamp.
I considered purchasing a SIT3 recently, but that factor made it a non-starter for my system. I went with a Pass Labs XA25 instead, which I have been delighted with.
Regarding the optimal ohm rating for the SIT3, the following is from Nelson Pass.
“The most popular impedance for the load is 4 to 8 ohms, which is the region where it measures best, and there is not a lot of difference between those two numbers. At lower, we see increased distortion, and at higher impedancethe distortion numbers don’t go up, but there is an increase in higher orderharmonics. It’s not a big deal, but you can play with this by putting some resistance across the speaker. Typically people play with resistor values from 8 to 25 ohms, and since resistors are cheap I recommend that you get several values to try. The type of resistor is not critical, but should be ratedat 5 watts or more.”
I have Avantgarde duo XD at 18 ohms. I use a Zu Audio 10 ohm resistor across my terminals to give the amp a resistance rating of 6.42 ohms calculated.
Works wonderfully.