Ummm, 660 mph is 574 knots. The speed of sound at -60C is 573 knots. I'm having a little difficulty with the declared speed of either a CV880 or 990. Not the posters who have these handles, but the airplanes mentioned above.
There are a couple of determining factors about how fast an "airliner" can safely go. By airliner, I understand something the size of an F-28 or above, and a jet powered aircraft. In the old days, a DC-4 was an airliner, as was a DC-3...
Jet powered aircraft are usually far less efficient in design than the engines strapped onto them. What I mean by this is that the airliner can only go so fast before it approaches the speed of sound. Approaching the speed of sound, drag increases logarithmically, hence very efficient "streamlining" is required in the airframe design to minimize that extra drag.
The drag phenomenon makes itself known though prior to the aircraft going M1.0. Due to various curvatures on and around the airframe, certain airflows over the plane may reach local M1.0 even though the airplane itself is only doing, say M0.85. This becomes the Critical Mach number of the airframe.
Modern jetliners are now routinely flying at M0.86 (B747-400, MD11) to name two. Some (G-IV and Citation X) can achieve M0.90 or slightly more. Mind you, they might not be considered to be jetliners by some. I'm just using them to illustrate the discussion.
Having said all this, even when the airplane is cruising at it's fastest cruise speed, the engines are more than capable of providing more thrust. Most modern jet aircraft can easily exceed their MMO's in level flight, but at great cost and great danger.
So the best way to improve the cruising speed, i.e. obtain the highest MMO possible, is to improve the airframe design. Two noticeable means of achieving this are the incorporation of area rule (DC-10, MDll center engine nacelle design, B747-200 upper deck design) to delay onset of supersonic airflow, and drag reduction (winglets, optimum airfoil design/sweep). I say drag reduction because no operator is going to operate their jet aircraft above its best lift/drag configuration for long if they want to make any money at it.
There is a lot more to this subject, and I apologize in advance for not expounding each point in more detail. But I suspect some haven't even made it down this far...
In closing, other than the Concorde and the Russian TU-144, I would offer the two I mentioned above (747-400,MD11) as my answer to the posed question.
Hit me with your best shots!