Polk recommends 20-100W per speaker. Usually you want to lean towards the top end for the best performance, if possible.
TA2020 chip used in the Lepai amp makes max 18 watts at 4ohms, and 10W@8 ohms, but thats at 10% THD
, which is often how chip amps originally designed for car stereos are rated. At 10% distortion the amp is right on the edge of clipping into a perfect resistive test load. Into a reactive speaker load with some negative electrical phase you will get slightly less power at clipping. Any amp playing near clipping sounds horrible, unlistenable by our standards. It may also have built in self-protection system which shuts the amp off when it senses a threat. Near clipping there will be thermal events that engage the protection which sounds even worse than voltage clipping, so using a chip amp at full rated power is not really possible.
For stereo listening with good speakers you want to focus on the 0.1% power rating. For this amp, playing in a car running on the highway with alternator providing 14.6V the 0.1% power output is 11W@4Ohms, and 7W@8Ohms. Using a 12V wall wart power supply would be a little less power.
The Polk T15 has a nominal impedance of 8ohm. While probably dips below that at some frequencies, it won't dip too far, because it is an inexpensive speaker that will most typically used with inexpensive HT receivers which don't perform well with low impedance speakers. You might be able to find an impedance graph of the actual load this speaker offers somewhere on the web, or by writing to Polk. But it's safe to assume it is a 8 ohm speaker, and that you have about 7 watts per channel from the Lepai.
Since Polk recommends 20W minimum per channel, I'd say you are underpowered with the Lepai delivering ~7W.
Typical of car stereo chips, this chip actually has 4 amps inside which are already bridged internally so additional external bridging of the stereo channels to get more power is not possible. And since the T15 has only one set of inputs, it is not bi-ampable. So there isn't any way of supplying it with 2 channels worth of power to each speaker (if that's what you were asking?)
You can find a heap of used receivers on Craigslist that will provide 50W/ch for a very low price. They get tossed because they don't have HDMI, but the audio section is still good. Then you might get a remote and radio too.