The K2p are nice speakers and the installer if he was worth his snap on tool set would be making other recommendations other than the lc6. I certainly understand why he is suggesting the lc6 but it is not needed.

The idea of taking the six outputs from the amp (keep in mind the front channels are full range and the subs are crossed over on the range of 80-120 hz) to sum that all together and send out six full range outputs to one amplifier is a total waste of both his time and your money.

The only reason to do it was if you are bi-amoping the speakers and using multiple amplifiers where it would be nice to have dedicated outputs, which in that case the installer would want to bypass the factory amp insert a high end processor capable of slopes in the realm of 24db, Eq, time delay, spatial averaging, ect. Something the lc6 does not do, nor does anything from Audio Control.

I can't speak to the input stages of the Alpine amp, that is better left to them. However if you can confirm with them they use differential type inputs then you should be assured of noise free interface between the factory amp or the head unit when connected to the 4.150.

That leaves the issue then to sound quality and this is most likely the reason behind the lc6 suggestion vs the heavily used Pac SNI35 or other such step down converters. Converts in general have a high degree of roll off at the top and bottom of the frequency range, better ones have less cheap ones have more and that is why most systems that use them don't sound very good.

If the amp uses balanced differential inputs then you can take the factory amps speaker outputs and run them directly into the amp. depending n the input voltage restrictions you will either have to use a speaker level input or sulfur on some RCA connectors and go in on the low level side. The only difference is the internally the amp is stepping down the voltage in line with how the amp company needs it to be to sound and work properly.