Any reason for poor HU performance?

I have an Alpine CDA-9811 HU, and for some reason, for as long as I can remember, it's sounded terrible at anything past 2/3 volume. Any bass note dims the lights in the dials and just sounds like it's killing the deck. Were HUs just not meant to be turned up that high without an external amp? It has a 4 channel 23W RMS (50 max) amplifier in it, which is pretty decent for an HU, but it still strains to do door speakers.

What speakers are you using?  What vehicle?Support the12volt.com

Sounds like it ain't getting enough juice - are you using the stock wiring assembly? Car details would be helpful.

It's a 1996 Mazda Miata. The speakers are dayton audio 8 ohm woofers with 6 ohm tweeters run through a crossover. It also did this with the stock speakers, which for some odd reason consisted of a 2 ohm pioneer woofer with a 4 ohm tweeter in parallel. I thought maybe I blew the head unit unknowingly using it to play such low impedence speakers, or that the old speakers sounded so bad because they were too low impedence for the deck, but I tried the new speakers on another channel and it sounds the same as always. I even tried hard-wiring the deck to the battery, same problem. I tested the voltage at the deck... A constant 14V with the car running.

Sounds like you are simply expecting too much from HU power.  The part that intrigues me is you say the HU is causing lights to dim?  This is why I asked about the speakers, as driving too low an impedance load might cause all sorts of HU issues.  Perhaps driving the OEM speakers damaged the head amplifier as it is designed for no lower than a 4-ohm load.  Your fault description indicates either faulty HU, faulty wiring or faulty vehicle electrical system.  You may want also to get your alternator and battery tested at a car electrical shop.Support the12volt.com

Well, it's the HU lights that are dimming, nothing else. Indicating that it's working pretty hard. I'll probably just have to go with a separate amp for the performance I want. Any opinions on the Alpine MRP-F250 as a nice basic amp briged to 2 channels for door speakers?

If the HU lights are dimming, I bet your HU is defective.  I do not normally recommend any 4-channel amp to be bridged to drive the stereo main set.  Too much distortion that way.  Use only 2-channels of a 4-channel amp or get a stereo amplifier.  Alpine amps are decent.Support the12volt.com

But pretty much everything in Alpine's current lineup is a four channel amp brigable down to two. Would you really suggest insisting on only a two channel amp?

Does it have to be Alpine? Lots of respectable & affordable 2 ch amps out there.

Well, what would you guys suggest?