I have no problem receiving LA channels, and I am over 70 miles away!
ABC is the weakest out of all of them
Where are you located?
We should be considered very lucky.... all of our channels transmit from the same location... and they are on tall towers on the top of a mountain. This means that they will travel a LONG way.
Here are some things to consider:
1.) Antenna height (get it up in the air as much as you can)
2.) Antenna Direction (spend some time pointing it)
3.) Antenna gain (get the biggest that you need)
I found that the pre-amp DID NOTHING for me... the gain of the antenna alone was sufficient to give me a good signal. ( I am 70 miles out, and I have about 50 feet of coax)
Don't waste your money yet, I am guessing that you probably don't need one. How long is the coax from your antenna to your TV (or sat box)?
Preamps can make things worse... remember that they will amplify any signal, include noise.
By the way.... yes some of the LA Channels are going to VHF and some are staying on UHF...
I would recommend buying a VHF/UHF antenna.
Since it sounds like you are having issues with the one channel, maybe you need an antenna with better gain or with better front/rear ratios. Since you are going to need to receive VHF this may be the time to upgrade.
Like I said, I am 70+ miles out, I have the largest VHF/UHF antenna that winegard makes (the HD-8200p it's huge.....14 feet long).. and I get CBS/NBC at 90+, FOX is in the 80's and ABC is around 70-80 depending on the day.
As far as combing antennas.... for the most part you will need to combine similar antennas (so connecting your indoor to you outdoor antenna probably won't work, and might make it worse)... and there are a lot of rules that go along with it....... The antennas need to be properly spaced, aligned, and connected. You will probably see much better results from getting a high gain single antenna.