Hi TEN_K,
You might receive more responses if you had posted your inquiry in the Gear>Hardware subforum. Maybe a moderator will move it there for you.
Recently went through a very similar selection process, so can offer some comments based on my own experiences.
First off, you should really review the practical audio interface latency info at the DAWbench site here:
http://dawbench.com/audio-int-lowlatency.htm http://dawbench.com/audio-int-lowlatency2.htm and also the very latest interface comparison data here:
http://forum.dawbench.com...-Performance-Data-Base Also, while you have indicated that you want a rackmount interface, you might also consider the option of mounting a non-rack-able interface securely onto a rack drawer, which would broaden your choices. I do this with other gear and it works fine. Plus, some half-rack wide gear is even designed to be easily fixed onto a rack shelf/tray.
Now, as for USB, I've not personally found any rack-able low latency USB interfaces, although reportedly the Roland Octa-Capture may be ok in that regard, dunno, DAWbench hasn't yet been able to get any Roland units for definitive testing, so the jury still waits.
The Presonus Audiobox 1818VSL was recommended to me by someone here as having low latency, but I found that it really did not, at least not with the current Windows drivers. You can see the data at DAWbench for the 44VSL, which uses the same drivers.
The RME Babyface is a desktop USB unit within your budget and it sounded ok when I tried it out, plus RME's drivers and latency are usually best in class, but it bothered me that the monitoring reverb fx was not done by onboard dsp but in the host pc. There are comparably priced desktop units from MOTU and others (Track 16, Forte), but any of them would need fixing down onto a sliding rack drawer as the controls are on top, so maybe not the best for a rack.
I don't know about the Tascam 1800 USB, although I am using the new Tascam iU2 desktop interface with an iPad, and it works fine also over USB with my DAWs. It is "USB compliant" so no special drivers are required, it just uses the built-in Windows USB driver. So maybe any newer Tascam USB interface might be the same, although I might avoid any older units that required special drivers.
Of course, USB performance may also depend upon the USB ports or chipset on a particular pc, dunno, I'd avoided older USB gear in past due to too many issues but haven't had problems with any recent USB stuff.
So, basically, for low latency rackmountable I/O options within your budget, the choices would likely come down to either Firewire (or proprietary over FW cabling), or ADAT connectivity.
From your sig, I see that you're using an H61 motherboard without onboard Firewire. So you should probably be looking for a PCIe card rather than a PCI one, because the H61 chipset lacks "native" PCI support and thus the PCI slots on your motherboard are actually "bridged" off PCIe, which can cause compatibility issues with many PCI cards (although perhaps you've been lucky and avoided any problems so far).
With a Firewire card (with TI chipset) installed in your pc, you could choose among a number of rack-able Firewire interfaces with fairly low latency (see DAWbench), including a Digidesign Mbox Pro (I liked its inserts). Used, you might find a good rack Firewire interface fitting both your budget and requirements.
Having a laptop-based Firewire rig, I'd been hoping that TC might update the drivers for their Konnekt interfaces, which really offer a lot including some nice onboard dsp fx (important for my purposes), but alas they still haven't done so yet, so I recently went for a Focusrite Saffire Pro 24 DSP instead after getting DAWbench confirmation of much-improved latency with latest drivers.
I don't know why you've indicated "very cautious" for the MOTU 2408 Mk3, because ime the 2408 Mk3 with a 424 PCIe card offers very good low latency performance, plenty of I/O expansion, current drivers (for Win7), a versatile DSP mixer and runs stably, plus it can be expanded with up to 3 more rack units, and requires only a single PCIe x1 slot. The connection between card and box is via standard Firewire cabling, which is convenient. The latency data at DAWbench is the same for the older PCI and newer PCIe versions, which is interesting in that the 424 PCIe card actually just uses an onboard TI PCI bridge chip for interfacing basically the same guts of the older 424 PCI card with the newer PCIe bus.
Another option you might look at is the ESI Maxio32 PCIe which offers a similar array of I/O as the MOTU and also includes some mic preamps. But I would urge caution, as it is not clear whether that unit is still being supported. Uses same TI bridge chip as the MOTU 424 PCIe.
An ADAT option is very flexible in enabling you to choose among a variety of outboard rack preamps and AD-DA according to your needs and budget, and while the MOTU and ESI units mentioned above do offer multiple ADAT I/O for that very purpose, it would also be possible to go with a pc-based ADAT card like the RME HDSPe AIO which offers 1x ADAT I/O along with analog and s/pdif, although it's beyond what you want to spend, as is also the 4x ADAT HDSPe RayDAT card which is RME's 2-card PCIe replacement for the HDSP 9652 2-card PCI version.
There are also Firewire ADAT interfaces from M-Audio and Presonus, but they are no longer in production and future support may be an issue.
The MOTU 424 PCIe + 2408 is what I ultimately ended up going with. But my decision was based in part upon already having a 424 PCI card + 2408 and existing ADAT preamps and AD-DA, so I wanted a solution which would allow me to continue using my existing 2408 and ADAT gear while keeping my future expansion options open and avoiding PCI slot compatibility issues when upgrading to a newer system. Your priorities will likely differ.
No doubt others here may be able to offer further input.
Btw, Vin, the operator of the DAWbench site, is a DAW builder in Melbourne, and I'm sure could answer any questions you might still have.
Hope that's helped more than confused!