I have sonar platinum using scarlett 6i6 2nd gen. In preferences should I be checking MME or ASIO. Have Windows 10 - 64 bit. Also, The default was MME. I can hear playback through speakers but not during peview (Headphones in 6i6 works for preview though)

Asio, without a doubt
 
You'll get much lower latency

ASIO or go home.

ASIO!
 

MME is just there for backwards compatibility with older audio hardware as well as built-in audio hardware, such as on laptops. Although SONAR now provides WASAPI for that under Win 10. For professional audio interfaces, always use ASIO.
 
Scott

--
Scott R. Garrigus - http://www.garrigus.com
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Hi. I will pile on with the "use ASIO". It is by far the better choice.
All the best.

MME was invented by the Etruscans circa 500 B.C. It served them well until around 300 B.C., when the Romans added refinements that have endured to the present day.
 
Seriously...the only use for MME used to be with laptops that had no external interface. ASIO4ALL was introduced as an alternative that worked well for some but not for others; it was highly system-dependent. With SONAR now supporting WASAPI, MME no longer has any relevance for SONAR users who have a modern operating system.

Anderton
MME was invented by the Etruscans circa 500 B.C. 

a6strng
I can hear playback through speakers but not during peview (Headphones in 6i6 works for preview though)

Do mean speakers connected to the PC's onboard sound system? Once you've selected ASIO driver mode and enabled the 6i6 I/O drivers in Audio preferences, you should hear all output from SONAR through the 6i6 - whether via headphones or line out to powered monitors or amp+passive speakers/monitors.
 

I use MME with my State of the art DAW and it works fine.