Not sure which monitors fit the Impress, but in terms of quality for 15k, it is hard to beat a Nanao MS8 or MS9
I agree with this! and as Aurich said, the non-tri sync models tend to look better with 15KHz games.
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Maybe you understand arcade resolutions, but in case you don't: There are many different resolutions that arcade games run at, but most CRT resolutions run at one of 3 horizontal refresh rates or "sync modes".
15KHz aka "standard resolution" this can be 240P or 480i or a host of other resolution, I'd say the vast majority of arcade games run at this resolution at least the vast majority through the 80s and 90s.
25KHz aka "medium resolution" this is mostly found on 90s era racing games, And As Mitsu said... Sega Model 2 and Model 3 and some atari stuff used it as well.
and
31KHz aka "VGA" or "high resolution" this is usually 640x480 or 480P This didn't start getting used until the late 90s and is used by NAOMI and System 256 and many games designed for LCDs can support this resolution in order to run on a CRT cab.
"Tri-Sync" means that it supports all 3 of these sync modes. you'll see some older monitors called "dual-sync" meaning that they usually support 15KHz and 24KHz but not 31KHz. They're great for versatility but because they support high resolution mode they often don't have the nice fat, razor sharp scanlines that you'd get with a 15Kz only or a dual-sync.
CPS2 and MVS are both 15KHz only so you only need a monitor that supports 15KHz.
The Nano MS8 and Nano MS9 monitors are arguably the best low-resolution monitors ever made (I'm also partial to the Wells Gardner k7000, though I don't think that came in a 27in)
the MS9 is a better option than the MS8, they're easier to source and a newer monitor. MS8s had high failure rates on the flyback transformer too; which is a part that is no longer made so yeah...
Those makvisions you have are likely tri-sync. I'm sure there are lots of people in the MAME world who would trade you an MS9-29 for one of them in a heartbeat as a tri-sync is really ideal for a MAME setup due to is wide resolution support.