What prog. do use for Pre processing

I use SharpCap and PixInsight and sometimes AstroPixel Processor.  I have had no issues with star colors or  distortion using this camera except for using the Celestron .7x reducer on my C11" EdgeHD.  However,  that  is not specific to the ASI2600MC pro.

I use John's method above (APP to stack and stretch, remove LP, and then Pixinsight).   Another great feature in APP is the star calibration tool, very easy to work with once you have taken out the light pollution, by selectiong the entire field (boxing it) and activate.  It provides a much more corrected field of colour.  This is testable by using the calibration tool in PI in a parallel process - and I have found the two to be very similar.   In series eg APP's calibration followed by PI's version, the second calibration is asked to do almost nothing, ergo the correction is already optimized.   You can PM me if you like with any questions. 

Cheers,
Ian

I use PixInsight for all of my image processing including calibration, stacking. etc.

I have the same camera and a C925 with f6.3 Starizona reducer. I use DeepSkyStacker for calibration, registration and stacking. It is easy (if you want you can load all your darks, lights,flats, etc and click stack to do everything, no other input needed), version 5 is super fast (200 large images takes maybe 10 minutes from calibration to stacking) compared to other software I have tried. This means if there is a problem very easy to adjust and rerun). It also has nice features like sigma-kappa rejection, etc, hot pixel removal, and comet stacking. It handles meridian flip with no problem, you don't need to do anything. I have noticed no issue with stars due to DSS. Here is a recent example: 
https://www.astrobin.com/full/yzxzpf/0/

Other nice features it provides offsets, number of stars, background level, FWHM for all subs that are registered, so you can use that to screen out bad images, and you can click on any sub to preview what it looks like.

The only thing I know it can't handle is different plate scales.
Rick

I  have been using APP for stacking, light pollution removal, star color correction, extraction of HA, OIII, & SII and then recombining them to produce OSC SHO images from dual narrowband filters. Photoshop is also used extensively in finishing, as well as the Apple image tools on the MAC. ASTAP is also useful, especially for tilt analysis. StarXterminator and NoiseXterminator are also very useful.