Posted this on The Gear Page... but wanted to share it here:
 
Recently picked-up a Presonus Quantum (audio interface)... as I was looking for something that had ultra low round-trip latency. 

Using Studio One v4, I loaded Helix Native... and created a patch based on the "Placater" (Friedman BE100).

• Set the Placater model's gain to 8 and the tone controls pretty much flat
• Loaded a stereo pair of my own Cab IRs
• Enabled a Reverb

Wow! The sound was stellar. No extensive tweaking/etc...
I owned Helix Floor for about 2 years. 
Was pretty happy with it, but I don't remember it sounding/responding this good. 
In literally a couple minutes, I had a Marshall style heavy-crunch tone that played/sounded as good as (if not better than) any of the top-tier modelers. I've owned them all...

With the Quantum set to 32-sample ASIO buffer size/96k, total round-trip latency was 1ms.
Super tight timing... and audio completely glitch-free.
Have to give mad respect to Line-6!  (Not for the low RTL but for the sound)
What started as more of a stress-test for the Quantum... ended up totally throwing a monkey-wrench in my work-flow. 
I don't know if it's the 96k sample-rate, the A/D D/A on the Quantum (good but not amazing), or the 64Bit double-precision float summing in Studio One... but Helix Native sounds/responds incredibly good.

Note:
You've got to have a fast machine to effectively run this type of load at 32-sample ASIO buffer size.
This machine was running a 8086k with all six cores locked at 5GHz.
 
The Quantum reports Round-Trip-Latency accurately. 
RTL was confirmed using RTL Loopback Utility (Oblique Audio). 

• 93-samples
• 0.93ms

You have me curious. Presonus dropped a major overhaul to the audio engine with 3.5, so makes me wonder how much that is playing into things. If you get a chance to try that setup in another DAW, the SO engine component would be nice to know or get a feel for.

It's not just Studio One.  

• You can do the same in Reaper (CPU use is significantly lower in Reaper).
• You can do the same in CbB.
• You can do the same in Cubase v9.5 (CPU use i higher)

ProTools 12 won't let you use an ASIO buffer size smaller than 64-samples when working at 96k.
 
What Presonus did with Studio One v3.5+ is implement a "Hybrid buffering" scheme.
Tracks that are merely playing back are processed with a larger buffer size (much more CPU efficient).
Tracks that need to be monitored in realtime are processed with small buffer size (low round-trip latency).
Best of both worlds...
 
BTW, "Hybrid buffering" is nothing new.
Logic did this *years* ago... as did Samplitude. 
 

Impressive.

Wow, indeed!

Hey Jim, how do you find the Quantum interface compares to the RME UFX+.  On paper it looks pretty nice and a whole lot cheaper. The main advantage I see with RME is that it goes with USB3 or Thunderbolt 2.  Since I got the upgraded laptop with Thunderbolt now, that part really doesn't matter so much.

Hey Jim. I have owned a 44VSL since getting my DAW going here (6 years now). My roundtrip at 128 samples 24bit 48khz. is 7.3ms.
I'm starting to think about upgrading my interface because it is no longer supported, and Cakewalk will not work here with Win 10 (7 still works just fine) at all.
Got any advice?
I'm glad to hear you're digging Helix. I still use a Amplifire 6, or S-Gear.

Gonna seriously consider this as my next upgrade, was looking at upgrading my MOTU interface with another MOTU, but this is intriguing to me. More for the money, it seems. Any comparison with say, MOTU 624 (Thunderbolt 3 and USB 3) or other MOTU devices. Or RME UCX or UFX? Thanks.
 

Genghis
Hey Jim, how do you find the Quantum interface compares to the RME UFX+.  On paper it looks pretty nice and a whole lot cheaper. The main advantage I see with RME is that it goes with USB3 or Thunderbolt 2.  Since I got the upgraded laptop with Thunderbolt now, that part really doesn't matter so much.

Hi Genghis,
 
The Quantum's forte' is lowest possible round-trip latency.
It has zero onboard DSP for mixing/routing.  Thus, all monitoring has to be done via software. 
There's also no means of (internal) loopback recording... you have to physically patch S/PDIF output>S/PDIF input or do the same with Lightpipe.
Otherwise, there's little to criticize.  The converters sound pretty good (especially at ~$900).
The ASIO buffer size can be set as low as 16-samples. 
Right now, machines aren't capable of sustaining much of a load a 0.5ms total round-trip latency.  
But... the setting is there.  As machines get faster, you'll ultimately be able to run at 0.5ms total round-trip latency.  
 
Presonus chose to strip Quantum down to the bone.
It's simple to use, the drivers are rock-solid, and it yields super low round-trip latency.
 
There's nothing to dislike about the Fireface UFX+ (other than the ~$2800 cost).
Total-Mix is extremely flexible.  If you want/need onboard hardware based monitoring, this is a major advantage over Quantum.
If you're moving the interface between machines, the Fireface UFX+ supports both USB and Thunderbolt connection.
 
As far as the onboard preamps, I'm going to call that a wash.
Both are neutral sounding... and yield pretty decent results.
If you're used to higher-end outboard preamps, they're not going to be at that level of quality.
 
For those who want lowest possible round-trip latency, it's hard to beat the Quantum (especially at ~$900).

clintmartin
Hey Jim. I have owned a 44VSL since getting my DAW going here (6 years now). My roundtrip at 128 samples 24bit 48khz. is 7.3ms.
I'm starting to think about upgrading my interface because it is no longer supported, and Cakewalk will not work here with Win 10 (7 still works just fine) at all.
Got any advice?
I'm glad to hear you're digging Helix. I still use a Amplifire 6, or S-Gear.

 
Hi Clint,
What's kind of funny about this... is that (to my ears), Helix Native (under this configuration) sounds better than Helix Floor.  Don't know what the delta is... but something is making a positive difference.
I was originally much more focused on the Quantum (to see if I could effectively work at 1ms RTL)... but Helix Native sounded so good it really caught my attention. 
Placater amp, a stereo pair of my MESA Cab IRs, super mild compression, and a Hall type reverb 
There was zero fighting the high-mids.
Helix Floor has no internal metering.  I'm wondering if I was having an issue with Gain-Staging (causing the issue with the high-mids).  It wasn't awful by any means... but could sound a bit artificial compared to a real amp/cab.
May wind up getting another Helix Floor to compare side-by-side.
 
On to your question (sorry for rambling on)
For more elaborate audio interfaces: RME, MOTU, Lynx, UA Apollo, and Presonus Quantum
For a simpler audio interface, I like the Audient ID-14 and now the ID-44 (connects via USB-C).
Both RME and MOTU offer low round-trip latency via USB.  
If you upgrade the machine and have Thunderbolt-3, you might want to check out Quantum-2 (scaled down).

• Define the I/O that you need
• Determine the connection type you'll want/need
• Is onboard DSP mixing/routing important?

That'll narrow down the choices.