Adding amp to Nissan Frontier RF HU

I have a 2005 Nissan Frontier with the base stock audio system.  I have upgraded the stock 2-ohm speakers to 4-ohm Alpine SPS-69C3 6x9 in front and SPS-17C2 6.5 in the rear.

I bought the 6-CD MP3 in-dash changer that came with the factory premium Rockford Fosgate system off eBay.  The premium system comes from the factory with an amplifier.  I found that the unit does not have enough power without the amp (it's almost too low to even hear at highway speed).

I don't need a lot of volume, so I was about to purchase an Alpine MRP-F250 (40Wx4) amplifer to use the speaker-level outputs from the head unit into the speaker-level inputs to the amp.  However, I go to thinking that this head unit has a VERY WEAK speaker-level output.  Realistically, it's probably closer to a pre-amp output that uses speaker wires instead of RCAs rather than a traditional speaker-level output signal.  I'm afraid that if the amp steps down the weak speaker-level inputs it may not work well, and if I use a LOC it will step it down too much to use on the RCA inputs.

I would very much appreciate advice from those who know more about this stuff than I do.  I've learned a lot from browsing through this forum for the last few years.  You guys seem to be very knowledgable AND helpful.

Thanks for any help/suggestions in advance!