Using the fx loop on the Roadster, or any amp for that matter, introduces another path of interference between the preamp and the poweramp sections. Basically, the signal in any fx loop isn't going to be as pure as it would be without the loop. FX loops are great because some effects, to some people's ears sound much better in the loop than out front. What makes the Roadster's loop great is that it is a series fx loop, rather than a parallel one, that runs parallel to the original signal. The series loop is inserted in-between the different amp sections.
When you disengage the loop on the Roadster, you get the sound of just the amp, without the introduction of the altered signal path for your effects and extra fx loop tube. There definitely is a difference in sound, but whether you will discern the difference, or even like it one way or the other is totally up to opinion. Personally, I enjoy using the loop because I also get the benefit of using the SOLO boost feature and the OUTPUT control. With the OUTPUT control, you don't have to operate each individual channel's volume control with their respective volume and gain knobs -- instead, you can dial in the sounds you want for all the channels in use, then turn all of the volumes up or down using one single control. This feature is also nice in aiding the user to achieve better lower volume (think loud bedroom) tones and sounds.
Essentially, this is the gist of the series fx loop on your Roadster. Honestly, I go back and forth with my loop on my Roadster. Some fx I swear sound better in the loop, while the very next day I will swear sound better up front. As fickle as that may sound, what I am trying to convey to you is to experiment and use your own ears to figure out what you like best. Also, read the manual! You will learn a ton about your amp and its capabilities by doing so.
Good luck!
