Mixers

The issue you had points to something was not cnneced properly, or even bad cables. Bad cables can kill your hi end. 
I have used most of the major brands of mixers and there is very little difference in sound quality if any. People who say " wow I just bought this Brand X" and the pre amps sound 10x better than my old Brand Y mixer"  are not being very scientific. 
 
I was in the market for a small mixer so I went into a big store which had them all and asked for a pair of headphones and a SM 58. I was mostly just previewing the built in effects. All the boards sounded the same to me but the effects are widely different. I bought the Yamaha based on the quality of the effects and it's routing options. 
I have also bought a small Mackie mix 8  because it had the perfect routing for what I needed but has no effects. It is supper small and I don't always need effects for some gigs.  
 
I was in a band last year and we bought a 16 channel Yamaha which one the bang for the buck shootout at that point in time. It was a great board for being very simple to set up and use. 
 
I really like Soundcrafts Signature boards if I was shopping for analog That would be my 1st pick. Things are always changing and right now there are some great buys on digital mixers. They often give you more in/outs for the money than analog. But then you need the get the hang of mixing with software. 

Yes Cactus Music was a full service music business including retail, back in the 90's. I still ran some of it's service, live sound and recording when I joined this forum. I just last year had my last client for recording and all I continue with is playing live. I play with different groups and on my own so we are always upgrading stuff. The last band I was in had an vintage 1990's Mackie board( sorry I never remember model numbers, ever)  It didn't have enought Aux for our in ears monitors and the effects where pretty useless so I wentshopping , this was 2 years ago.I like chose the Yamaha only because for the price it had the correct in/outs and the effects were better. any how everything changes and Mackies are still great boards. 
The only way to see if those pre amps make a difference do like I did and go to a store and compare them. 
But I would guess the more you pay the better they will be,, If you can hear a difference: )  I would not be able to guess. My little Mackie Mix 8 must use the cheaper pre's due to it's very low price point but it sound real good to me. My acid test is to plug in headphones, and turn the gain up past 3 o clock.. Some boards get real noisy. My Mix 8 doesn't. I use it instead of my Scarlett pre's as example of better quality. 

If you look for a mixer also consider the latest behringer digital mixers. Extremely good value for what they cost and they come in many different sizes ...

Me too, I'm flabbergasted how much you get for the money. 4 FX buses, a main bus and 6 mono aux buses, plus an 18 channel interface with zero latency monitoring and 16 preamps.

It sounds pretty good live too.

I agree there!
 
Interesting trick (mostly for live) is that if you have channels to spare you can actually duplicate channels from a single input. E.g. set channel 1 and 2 both to input 1, then you can use one to feed the front of house and another one to feed the lead singer's monitor who might want their voice EQd differently than you want on the mains.