@olsenn @harrumph: By default both generic 4:3 and 256×240 line5x modes have 1920×1080 active area as seen by display: the sides are padded with black bars and top/bottom of the source are cropped a bit (“Line5x format” setting can be used for switching to full 1200p output). The aspect ratio in generic 4:3 modes should be equal regardless of linemult mode, but in 5x (1080p format) the vertical cropping may slightly affect that.
256×240 modes usually need sampling phase adjustment, see the wiki for details.
Selection between DVI and HDMI should not affect the aspect when using line3x or higher, maybe the TV has different processing path for “PC / DVI” video. You may try toggĺing “HDMI ITC” (while using HDMI TX mode) option to see if it improves clarity.