After RC Strings developed its high tension Sonata Forte strings, some soloists found that their guitar and their technical needs required even more tension. In response, RC Strings created the SX80 Sonata X Forte XHT classical guitar strings, which offer a new dimension in power and richness.
The strings, which are made in Spain, pair clear nylon trebles with silverplated basses. Like all RC Strings strings, they have an anti-corrosion treatment.
| Item # | Note | Gauge (in.) | Tension (lbs) | Gauge (mm) | Tension (kgs) | Material |
| SX81 | E | .030 | 18.957 | .75 | 8.617 | clear nylon |
| SX82 | B | 0.033 | 13.688 | .85 | 6.222 | clear nylon |
| SX83 | G | .041 | 12.665 | 1.05 | 5.757 | clear nylon |
| SX84 | D | 0.035 | 24.212 | .88 | 11.005 | silverplated wound |
| SX85 | A | 0.038 | 16.202 | .96 | 7.365 | silverplated wound |
| SX86 | E | .046 | 15.649 | 1.18 | 7.113 | silverplated wound |
