Need MIDI Guidance in DAW Rebuild

Why not just record tracks in sonar as synth tracks and plug in whatever vst you want. I don't but a lot of people use TTS to sketch out ideas then replace with other vsts later in the process. Not sure what you mean about FF400 and "synth engine"?? 

The much maligned Soundblaster series did have the ability to run soundfonts from hardware, which is capable of very low latency. That was something that was pretty useful in the days when we used machines with a gig or less of memory and a processor that ran a clock an order of magnitude ormore slower than today's machines. A modern machine should be able to run a rompler from software without excessive latency, and there are a ton of free soundfont players out there. Are you running a seriously antique machine? If not, then maybe you should look into why the latency is so high.

Ahh, so you already have replaced your soundblaster with a top notch RME interface, now we just need to know what your computer specs are to determine why you might have excessive midi latency when trying to create something.
Though, your post subject seems to indicate you are looking for advice on creating a new DAW, or perhaps just getting the most efficient midi usage from what you already have ?

I have both the RME and the SBLive in the system, which is a dual core Pentium 3GHz.

It's easy. If Sonar see your RME midi ports, then go to add track - instrument track - and choose TTS. Then in input signal choose your RME midi channel ( omni it's good for start ), and you will hear your TTS sound when you play on controller. Rest is for you, you can record, or change sound in TTS.

Midi make no sound itself, but if you connect your midi output from controller to midi input in softsynth like TTS in Sonar, then you'll hear your track. This is the way that we all go, midi composers😊.
What version Sonar you have?

You where experiencing latency most likely because your not setting things up properly and/or using the wrong drivers. RME is as good as it gets and you defiantly need to be using that as your DAW's audio interface and drivers.
 
You may even need to remove the Sound Blaster if it is a PCI card because they are well know to steal the drivers for other devices cause more trouble that they are worth. Creative drivers ( in the past) are notoriously bad and do not run smooth with Sonar. I know first hand as I have a Creative Audigy II PCI card. That was why I ended up on this forum in 2004, I couldn't get anything to work in Sonar. Things improved for me immediately when I switched to a proper USB interface. I installed the Audigy in one of my DAW set ups once and ended up having to remove it because it conflicted.
 
So make sure in preferences that the RME is your audio driver and clock source.
Make sure your using ASIO drivers and best if they are up to date.
 
Make sure your studio monitors are using the 1/2 output of the RME. You DO NOT want to use your computers output.
There is no reason that you should not hear a VST instrument and with zero latency.
 
There's a few other trouble spots.
 You PC is grossly outdated, you mention a re build??
Might be a good idea to post the complete system specs including RAM and which OS your running.
And which version of Sonar are you running on that?   
 
Also your RME is firewire, there are well know issues with having the proper Texas Instrument chipset when using Firewire.
 
 

I'm using WDM/Kernel drivers, not ASIO (although frankly I can't recall why).   The SBLive is not in the rebuild, yet.  That's what I'm now trying to figure out.
 
System is a dual core Pentium 3GHz, DDR2, XP.  So while not exactly "modern"it has plenty of horsepower.  More than enough for my needs.   And yes, using TI chipset on the Firewire card, never had a problem with the card or the RME.  Although I am probably going to try a PCIe card (also TI chipset) this time around.

I will experiment with the latency settings....

You still din't say what your RAM is and what version of Sonar. 
You are running a very outdated system and things have changed, Hard to trouble shoot with half the information. 
Start with switching to ASIO and trying a VST instrument and report back.