Mark,
I think the cheapest way to go would be to add just the Big Ben to your TNT 130 as an extension speaker cabinet. You will still be using the TNT as a complete combo, but now you'll have two speakers connected to it. This will give you more acoustic output, and more depth to your sound. Big Ben sounds real good in a full range application such as this. Then later, you can add your choice of many different components: a preamp, an amplifier, or a high range speaker cabinet. I would recommend that you look for a used bass head that combines a preamp with a power amp for the most cost effective approach. There are many choices out there now, and your ears will be the best judge. This of course means that you will have to test drive some amps in a few stores. If you like clean hi-fi sound, you can get an older SWR SM-500 (500 watts mono or 250 watts stereo)with built-in crossover, tube preamp and solid state power amp. This amp is real light, 2 rack space high, has a 5 band graphic EQ, effect loop, direct out, and Aural enhancer circuit. It replaced the SM-400 (400 watt) amp I own. They can be found used for around $800. give or take a few bucks. SWR also made an SM-900 which has 2 preamp channels, more power output, and conventional rotary semi-parametric EQ preamps instead of the graphic EQ of the SM-400 & SM-500 amps. Expect to pay a little more for this amp on the used market. The sound of the SWR is high tech, refined, clean and powerful. Very little background hiss. The tube preamp is really warm which is a good thing with an Alembic. I have never heard any Bass sound bad through an SWR amp. However, it is not the only choice. EBS makes a really nice amp. Eden, Trace Elliot, Ashdown, etc. Even Ampeg makes a few great amps that really rock. Once you add a Bass head to the Big Ben, you will probably want to look into a high range cabinet to satisfy all the other strings you play. A good cabinet to try would be a 4x10 design (Example: SWR Goliath) or even a 2x10 for ease of travel. Some 1x12 cabs are really nice too, and can fit the bill nicely. Bergantino makes a 3x12 cab that is really clean. Again, lots of choices.
If you want to go truly Component, then look into seperate preamps and power amps. An Alembic F1-X or SF-2 can be used with a power amp as a great rig. Many threads here in the club list a lot of players setups and you can get dizzy reading about them. I think you can take it a step at a time and really have fun too. I know that you poo-pooed GC earlier, but I've seen some really great used gear deals there. And the price on the tag is negotiable somewhat too. So haggle!
Kris