We received both the TSN-501 and TSN-553 samples today. I didn't spend much time with the 501, but did a fair amount of comparison between the 553 and other scopes.
Our first test pitched the Opticron MM3 50mm with SDL v2 eyepiece. The difference was beyond astounding. Since UPS got here around noon, long distance viewing was fraught with heat shimmer. We concentrated on holes in the hedge across the parking lot. The Opticron could barely see into the hole, but the 553 saw nearly to the far side of the hedge. The 553 also had significantly better contrast. Cell phone photos bore this out.
Later in the day, I compared the 553 with the original version of the Vortex Razor 16-48x65. Again, the 553 was brighter, sharper, and had better contrast. I didn't take any photos of this comparison due to time constraints.
These are preliminary observations. I'll have more to post next week. I plan on taking the 553 birding on Saturday morning.