The Emotiva USP-1 is the more cost effective of the two and I bet you couldn't tell the difference between the sound of either. I wouldn't worry about an output level control on a preamp, your UPA-1s will likely not damage your speakers. Too little power has the better potential of damaging speakers as an amplifier driven to clipping is more likely to cause problems. With the UPA-1s, the power to the speakers is necessarily limited by how much volume you can handle. Your speakers are rated to handle 100 watts, that doesn't mean you shouldn't use an amp with more power than that. A more powerful amp just means that transient peaks in program material will be handled better, resulting in better sound. I think you can hardly go wrong with the Emotiva USP-1.

Very rarely if ever is a speaker blown from too much power. In most cases it's blown tweeters. Tweeters blow because the amplifier is driven into clipping. On the rare occasions when a woofer is blown there is usually plenty of warning. The woofer will bottom out and make lots of un-musical sounds first. When a tweeter is blown because of amplifier clipping there is no warning only a sudden loss of treble.

More power is almost always better.