Plunder Their Supplies (52) Cemetary
Malruk Succubus (56) Dragon Valley
No XP-SP quests at that lvl on repeatable scale. At 60 go to Valley of Saints and take the quest from the Hierarch and the Necromancer, one is adena and one is xp-sp
An option could be enchanted vally ( quest from WH freigt dwarf ) for 100 jade crystals u get about 55k. The mobs also drop a lot spellbooks you can sell for 100-300k ( if u have the time for that). xp is good because of 2x and 3x hp mobs.
Else dragon vally with coin of magic quest and the DV quest you get a lot Gold wyverns and sometimes EW C or Materials from the other quest.
True Enchanted Valley is nice too, but it's not really a nuker solo spot, bit costly per mob it seems. But the Jade Crystal thingy is a good adena quest.
My favorite ones are the alligator pelts, teeth, and treasure maps in Heine. It's in the 40-52 range and you can get money from one quest (starts with the items shop trader), another gets money from the warehouse guy, and also sparks a quest about maps with the weapon shop guy that can yield EAD, EWD, EAC, (I think even EWC), money, gems, and sometimes even elven mithril gloves.
I spent a year there one month. One of my favorite packed hunting sites--cause the mobs are THICK.
Quests are okay and swimming for the trunk and killin the sharks is a nice diversion, too.
Prob a bit low for "mid 50's" of OP. My DA at 52 has almost everything green to him (although since they're high exp mobs he struggles to kill multiples when he's solo).
I love those quests! My ws will start running them before long :)