Looking to redesign my rig..
 

I've been playing in a group that keeps evolving. I have already made a ton of gear upgrades, but this is where i'm at:

MIJ Fender Jaguar, passive circuit.
ProCo Rat with LM308,
Ampeg SVT 4 pro
BBE Sonic Maximizer in the effects Loop.
Ashdown ABM 8x10

Right now the music we are writing is the newer kind of "clean/heavy" sound. Its a 3 piece with no vocals, we all share influences like Underoath, Decoder, Oceana's Clean Head ep, and Circa Survive.

Now I feel like the part of my rig that im least happy with is either the cabinet or possibly the bass. Would adding a 1x15 be too much with out cutting down to 4x10?

Adding a 1x15 might not be 'too much' if you can deal with hauling all of that stuff around, but most people will say that mixing cabs like that will probably lead to problems. It might sound alright until you blow the 1x15 because it can't keep up with the 8x10. What are you unhappy with?

I see no problem with your bass, and adding a 115 to an 810 is like adding a battery powered fan to a jet expecting it to go faster.

What is it about your sound that you dont have right now, that you want? The first thing I would do is to take the effects out of your signal chain and just listen to your bass, amp, and cab. Make sure you have set your gain up like this, (i.e. crank the gain and bring the master down to suit). Play with the eq, dont let your bass knob go above noon, and try dialing in some low mids. Can you get close to the sound/distortion you want with just your amp? If you still feel you need more add in the pedal. Put the sonic maximizer up for sale, it is a tone suck.

If you still are not getting where you want to be tonally, then you might look at a different cabinet. There are much better options for "low and clean".

I've considered moving to a 2x10, or a 6x10. Mainly because I think that the SVT 4 pro gains a lot of depth from bi-amping. A friend of mine plays similar music and biamps to a SVT 1x15, and an ampeg 4x10hlf. I think it was the extra punch that I dealt from the 4x10. He also uses a grabber, so that accounts for a substantial amount of tonal difference..

I really like the ProCo with just a touch of distortion, and the filter maxed, giving it a fuzzy feel.

How can I get more punch, without losing too much of the jazz PU clank?

Thats not the Bi amp, thats the HLF adding the lows. It is one of the lowest tunned commercial cabs. In fact I would suspect the 115 is filling in the holes in the mids as well.
Cut some sub lows and add some low mids around 100-150hz. I have not used your particular 810, so I dont know if its natural voicing is fighting you. I WOULD suspect the sonic maximizer is killing your punch as well as it is a massive mid scoop.

So the maximizer would help me more if in needed more clank and less punch?

I read in another post on getting a clean tone that I should max the volume on the Graphic EQ as well. Is that where I should start after the main preamp section?

There is no good way to use the maximizer. Seriously take it out of your rig for a month, and see if you miss it.

No you do not want to max the GEQ, setting your gain properly will take care of all the volume concerns, and will also warm up your tone in a huge way. If you are going to use the graphic, start with it flat, and as you are playing slide each control all the way down, then all the way up one at a time. I would look to end up with some sort of cut below 100 Hz, a boost between 100-250, and then very slight cuts if anything above that.

A few examples:

http://www.youtube.com/watch?v=TOdfV1EjUgg
http://www.youtube.com/watch?v=R7edFefqsCE
http://www.youtube.com/watch?v=BML6LJM-lwA
http://www.youtube.com/watch?v=cOZlAunMb8o

Pretty much the top songs that show up on YT for the bands you mentioned. In all these cases, these are in the fender/ampeg wheel house. You could cover this tone with out any additional help to your SVT. As I said the first place to start is to just listen to your bass, amp, and cab. Set the whole EQ flat and get your gain up, maybe 1-2 o'clock or higher depending on how hot your bass is. Tweak the EQ from there.

Most helpful post on TBass

The SVT-4 gets a bad rap, it is based on user error. It is actually a wonderful amp if you have the gain/eq set correctly.