Sub and Amp questions

I have a 400 watt 4 channel Jensen amplifier that I am going to have installed in my 1987 BMW 325es. I don't know that much about car audio, so forgive me if my questions are stupid.

1. Is it necessary to buy a sub to go along with the amp in order for it to work? Or will it just make it sound better if I get a sub?

2. If I do buy a sub to go along with my amp, what would be a good brand and wattage? I am looking to spend anywhere from 100-300 dollars max.

1.  You have to have some speakers connected to the amp to get any benefit.  ;)  You didn't mention speakers.  Are you replacing the factory located speakers?  And, no, it's not necessary to have a sub for the amp to work.  But a sub will fill out the sound with the deepest bass tones.

2.  First, need to know what speakers are already planned for use with that amp.

What is the reason for installing the amp?  To blow out the factory speakers?  You would have about 50 watts going to each speaker that was built (15 years ago) to accept probably about 10 watts.

Is the amp being installed by a pro?  And what is his recommendation?

Sorry, that sounded harsh :)  I recommend that you plan on purchasing replacement speakers for the front doors.  Check Crutchfield for "what fits my car".  Then shop the internet for a good deal on a pair, 50 watt RMS.  Hook those up to two channels of your 4 ch amp.  A good sub that you can get cheap is the Cerwin-Vega from millionbuy.com.  About $50.  If you can build the enclosure for it, its a great deal.  An easier solution would be to buy this kit  https://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=260-580  for the subwoofer.  It could be connected to the 2 other channels.

There are other necessary things, too.  The radio (head unit) is usually replaced as well.  But original head units sometimes are kept in place and an adapter is used to transfer the music signal to the amp.

Is that a new amp?  Find out all you can about it, impedence rating, bridgable?  Spend some time reading through posts on this forum and you'll find a lot of info.

I am having a professional install the amp for me. There's no way I could do it, lol. I will strongly consider buying new front speakers and those subs you mentioned look like great deals.

Thanks for all the helpful tips and info!