My first thought is, you're doing extremely well. Reception of distant signals is expected to be a challenge as you're describing. You're dependent on the atmosphere to act as a reflector to guide signals... but because atmospheric conditions change with time of day and as weather systems move through, you have no control over one of the most important parts of the signal path. There are also formidable mountain ridges blocking and scatting signals from the LA area.
I expect the CPA-19 preamplifier will help by eliminating the effects of the cable and splitter losses in your distribution system.
Go ahead, try the CC-7870, it's not too expensive and if it does what I expect, can be removed easily. The problem most people encounter with unfiltered antenna combining (simple reversed splitter and phasing harness methods), stems from the fact that both antennas receive signal from directions other than the few you would prefer. When the output of each antenna is mixed together, the signal from one antenna may add positively but on average, in half the cases, adds negatively and has the effect of being an interfering signal. Then in most cases, the antenna aimed away from a station, also receives a strong version of multipath reflections which increases the chance that the combined signal will be lower in quality if not lower in level.
In the cases where co-channel interference is an issue, the signal from an antenna aimed away from the desired station is far more likely to cause trouble when combined with the preferred antenna output.
If I were going all out, starting from nothing, The DB4e would be an excellent antenna to face southward. An Antennacraft Y5713 would be my choice for making the high-VHF signals reliable. I would combine the two antennas with a UVSJ and try running with no amplifier. If one proved necessary, an Antennas Direct CPA-19 or Channel Master CM-3410 would be on the short list. For the LA signals I would consider using a long boom Yagi such at the Antennas Direct 91XG or a Winegard HD9095P... plus an Antennacraft Y10713 each fitted with separate CPA-19 preamps. I would combined the amplified outputs with a UVSJ. Because this is a 'dream' system, I would mount all of this on a tower that clears all obstructions, and rotate the LA/DX antennas using a HyGain AR-40.
There are several approaches to solving the signal combing problem:
http://forum.tvfool.com/showthread.php?t=820
http://forum.tvfool.com/showthread.php?t=2882
http://forum.tvfool.com/showthread.php?t=8275