It seems the gain of the amp is not a published specification. I can only see it inside the Stereophile review under measurement section.As a rough approximation, you can calculate the gain by using the input sensitivity and output power numbers, and then adding a few db to the resulting number:
First, convert the 8 ohm power rating into volts, based on
P = (Esquared)/R,
where P is power in watts;
E is the voltage corresponding to rated continuous power into 8 ohms;
R is 8 ohms.
In this case, P is 150; R is 8. Therefore E = square root (150 x 8), or 34.6 volts.
Using the 1.5 volt sensitivity rating that was provided above by Dave, the voltage gain is 34.6/1.5 = 23.1.
Next, convert that gain to db by taking 20 times the logarithm of the voltage ratio:
20 x log(23.1) = 27.3db.
As I indicated, the actual gain will be somewhat larger than that number (in this case by 5db), because the continuous power rating will be lower than short-term power capability, and because of margin that is provided in the specs.
I would like to try the SET amp.... Does the requirement to have input impedance > 100k Ohm still apply?Yes. That is a consequence of the preamp's output impedance characteristics. As for specific amp recommendations, you might want to research past threads to see what amps people have used with the particular Zu model, and/or start a separate thread on that question.
I can lower my DAC input so I can make the volume know of the preamp within a usable range. This is similar to if I lower the volume in iTunes. My question is, how would this affect the sound? Will I run into the bass roll off problem like using attenuator?IMO reducing the setting of that volume control is a reasonable thing to try. The bass rolloff effect I described has no relevance to that. As Ralph (Atmasphere) indicated, the sonic effects of reducing the setting will depend on the specific design, and only you can make that judgment.
Regards,
-- Al