If you want to do things on the cheap your best bet is to buy a 2 channel amp and run the subwoofer bridged over the two channels. If your HU does not have a dedicated subwoofer output where you can vary/limit the output frequencies then you will need to make sure the two channel amp has a variable crossover built in.
If you are feeling rich and want the best then I agree with Britto_1.3Si a Mono Amp is the way to go as they are designed to be used with a subwoofer. Then you should upgrade your front speakers and power both the front and back off one 4 channel amp or two 2 channel amps (better IMO).
However your HU will power the front and rears fine for now at 45wpc RMS. So if I was you Id buy an amp for the sub now and see if you are happy with the sound from the fronts and rears once the sub is installed.
Dont forget that installing a sub means that you no longer need to output all the low frequencies to the front and rear speakers allowing you to go louder without distortion (especially if not using component speakers) while also being able to fine tune the sound more to your liking.
Ive found
www.caraudiocentre.co.uk very reasonable and you have more 'come back' with them if you get into bother than with most cheaper ebay sellers.
Im not saying that a 4 channel amp would not work for powering rears and a subwoofer, but...
- has anyone had any real success doing this?
- Can anyone recommend it?
I guess as long as the amp has two separate sets of phono inputs (in this case for rear and sub) and independant gain & volume levels for each input. It could be a good compromise if space is a serious issue or if one 4 channel amp can be obtained at a significant saving when compared to two 2 channel amps.