I guess all along I have expressed a desire that the JH13 had a touch more bass. The TTVJ Portable fixed that for me. But it is a somewhat softer presentation, so while the detail is there, it's a bit more lush. The Slim, on the other hand, seems to have more of the HeadAmp sound. Very clean, very detailed (not bright), and lots of slam in the bass. But where the TTVJ is more "rounded" in its presentation, I found the Slim more "angular"--sorry I'm feeling inarticulate. Bottom line: for those who thing the JH13 have plenty of bass, then I think the Slim is going to make them very very happy. Mid-fi types like myself may want to find something that gooses that midbass a bit.
Also, this is all based on a very short time with the Slim, so I look forward to receiving my production version for further listening. Again, I can't believe how much I love the volume control. It is simply outstanding and, IMO, represents a real step forward in ampdom.