Ok, here's the thing to watch out for...
The drop amp you have has a rated noise figure of 2.4 db. That's actually pretty damned good for the type of service you're putting it to. The ONLY way you will make any improvement is to add an amplifier with a better (lower) noise figure than that.
That precludes a LOT of what's being sold out there for scanner users. You can put any old amplifier in front of your splitter and it'll make strong signals stronger, but at some point, it'll make weaker signals worse.
So, you want to actually improve things? Find a preamp with a noise figure of around 1 db or less, I'd suggest something from Advanced Receiver Research, and expect to pay around $120 or so.
If you spend less, expect a worse noise figure than what you already have, and expect it to not work very good.
Oh, and if you're going to put a low noise preamp at the antenna, be prepared for the thing to get overloaded if you don't have any filtering in front of it. What sort of filtering? You want to filter out EVERYTHING except exactly what you want to listen to. Try it without the filters, you may not have a problem.
And one more thing... If you put a preamp in front of the splitter, you'll probably have to put some attenuation between the preamp and the splitter to avoid overloading the already pretty decent amplifier in the splitter. Taking a 1 db NF preamp, and putting it in front of a 2.4 db NF amp is fraught with all sorts of potential problems, namely overloading the second amp. You'll end up wiping out the noise figure of the entire system and making it 10 or 15 db, which is horrendous. It's not unreasonable to find a 0.5 or 1 db preamp with 15 or 20 db of gain, install it right at the antenna, put 10db of attenuation after it, then your 40 feet of LMR, then the 2.4 db NF splitter. Then, you'll probably see an improvement.
Reality is, it's probably not worth the extra expense and trouble.
