Make sure your amp cables are ran straight to the battery. Also the ground needs to be bigger than the feed.
If they are ran to a distribution block the feed probably isn't big enough to handle the load. Your amp should run cooler also.

Thanks, another good fix for my audio guy to look at.

All good suggestions.. A capacitor(or 2) cant hurt anything if you cant get any more batteries into the system(they also act/are large capacitors) Cabling is a huge loss of power, larger is ALWAYS better when it comes to audio power cables. Another thing you can do while trying to limit dimming is make sure your alternator is up for the task. Ive had alternators rebuilt larger than factory specs to help with that as well.

IMHO: house battery keep as deep cycle. The batteries that are used to start the engines should be normal Starting / Cranking Batteries.

Also, the house battery should be isolated from the Cranking batteries.

Other experiences may vary.

The house battery is totally separate. Was told by mechanic to use deep cycle for engines, won't be first time he was wrong. Would have thought that they would hold up longer with no adverse affects.

Deep cycle for trolling motors. LOL.

Deep cycle is not for big block engine starting.

What are effects of running bbs with a deep cycle battery?

Think of a battery as a 5 gallon bucket full of water. A deep cycle is design for low-moderate prolonged demand..you have a hole in the bottom the size of a dime that water (electricity) slowly poors out of. A starting battery has a hole the size of a big gulp so water can gush out for a short period (heavy load by starter)

I would make the amp ground 200% of the batt feed side