New speakers make cassette sound bad.

This may be due to impedance mismatching and/or if the feeders from the amp are not wired correctly. My only reference if for the Gen III JBL system, so the info may vary. Your Fosgates are 4 ohm, the front Gen III JBLs are 1.8 and 2 ohm. Another thing to consider is the JBL door speakers put out more low end freqs. Wattage is not the problem. This may not apply to your Gen II system exactly, but should be close give you something else to look at.

 

Hi there, well first off, did you replace the speakers with speakers that have the same impedance as the OEM speakers? You need to maintain the proper impedance for the JBL amp to work properly. Look at the magnets on the OEM speakers you removed. There should be a stamped designator either 2 Ω , 4 Ω, 8 Ω, 16 Ω, 32 Ω
I think you get the idea, and see if it matches the numbers of the replacement speakers! If it does not there could be a problem!
You state the FM sounds OK? But cassette is bad? Did you try playing a cassette tape, did that sound OK?. If you have the Disc option, play one make sure it sounds fine. If the tape playback also sounds bad the tape head inside the unit may be out of alignment. You would have to pull the unit and get access to the head alignment screws! there are things to look out for in head realignment,
Azimuth is the alignment of the heads perpendicular to the direction of tape travel. If the azimuth is misadjusted, any program recorded on both tracks will be played back slightly out of phase.
Zenith is the alignment of the heads in the same plane as the tape. For example, the bottom of the head can be kicked out too far, which may cause the top tracks not to fully contact the head, and would cause the tape to ride up on the head, snapping back down occasionally. The zenith can also be misaligned so that the top of the head is shoved too far out. The head should be aligned so that the tape rides against it with smooth contact to the entire surface.
Wrap is the alignment of the head so that the tape contacts the head at the center, aligned with the head gaps. The wrap adjustment causes the head to swing in an arc centered on the middle of the head, and moves the center of the head from the left to the right of center, depending on which way you adjust the screw.
Head height is adjusted so that the tape rides perfectly centered from top to bottom on the head. You can tell this immediately just by looking at the tape as it rides across the heads.
Tape-to-head contact is just that - the head should be positioned far enough into the tape path to ensure good contact with the tape. If it were too far back, insufficient pressure would cause playback to be inconsistent.

Since you say the cassette adapter works OK in a different player, I think you might have Tape Head problems!

Let us know what you figure out!

 

KK6PD, how did you make the "ohm symbol" with your keyboard?

 

Ω Never mind. Hold down the Alt key and type 234 on the numeric keypad like so. Google is your friend.

 

I like copy/paste!!!