You don't want a flat or straight back. Too many hounds are so flat you could use them as a table. A flat back means that the animal is not properly built. The arched loins are there for a reason and the belly is well drawn up. It is interesting to note that the vertebrae of the loin are slightly wedge shaped to give added strength to the loins which have no skeletal structure underneath for support.
Muscle adds to the arch.
One often sees a flattish back on a young hound up to 18 months or so, he has not yet developed the muscle which will contribute to the arch. Ears, small, with greyhound-like carriage. That means small and rosed. That is also mentioned for a purpose. Lots of breeds used for hunting that did not have this small ear were cropped.